Initial Qt6 support

[adamlamar]

Fixes #1033
Fixes #365
Fixes #912

This PR implements compile support for Qt6. There are still some bugs and limitations, but the app compiles and runs. The changes mostly fall into these categories:

• Disabling deprecated APIs in 6.0+ in the Qt5 build
• Adding a few Q_OBJECT references per the clazy tool checks
• Some minor changes to string handling that usually caused ambiguous method references, or because a constructor was removed
• Handling of new methods or introduction of enum types (e.g. enums converted from int usually don't implement operator+)

A few more things to fix before this is ready to merge:

• The "more" menu (the ... icon) needs to be built
• The context menu for the webview no longer has access to createStandardContextMenu() - need to find the replacement
• The print method no longer exists on a webview page - need to write up an approach to write a temporary PDF to disk, then print the PDF
• Write up a README to document instructions for building with Qt6 on ubuntu 22.04
• Test the new keybind sequences on a non-VM

[kelson42]

@adamlamar Thank you very much for this draft.
@mgautierfr @mgautierfr Could younplease have already a quick look and give feedback about what has been done so far?

[adamlamar]

Some more updates on the context menu:

• I couldn't find much on the Qt6 docs, but AFAICT the createStandardContextMenu method is simply not available anymore. I decided to implement a custom QMenu and make it more consistent with typical browser behaviors
• When you right-click on the background, you get the standard Back/Forward actions. The new menu uses the same icons as the toolbar, instead of the previously used system icons
• When you right-click on an internal link, you get Open in new tab. This is also what happens if you middle click on a link (and this is what browsers typically do)
• When you right-click on an external link, you get Open in new browser window. This is what happened before
• There are no longer menu actions to Copy link or Save link, since I believe these were not intended (see screenshot below)

Old:

Here's a video with the new approach:

new-context-menu.webm

[adamlamar]

@mgautierfr I've completed the tasks above and AFAICT everything is working on both Qt5 and Qt6. Should be ready for a full review. Let me know if you have any questions.

[kelson42]

I had a brief look, here is a first set of remarks:

• Compiles great with qt5
• Instruction to move to qt6 are perfect and switch is smooth
• Compilation works even if IMO a few warnings are new and should probably be fixed (if possible)
• The library language filter seems broken for me with qt6 (after relaunching it seems the bug has disappeared):
  

[kelson42]

To me it fixes #365, #912 (with qt6.2 on Ubuntu), Could someone confirm?

[veloman-yunkan]

I only reviewed the code (didn't try to build and test kiwix-desktop with qt6).

[adamlamar]

@kelson42 I noticed those same warnings, but I believe they were preexisting. I actually have a branch where I fixed all the warnings and enabled -Werror, but its based on this branch so might be easier to wait until this is merged. I also noticed that library selection issue on first start - let me see if I can reproduce and figure out what's going on.

@veloman-yunkan Thanks for the comments. I will work on improving the code portability in those places where there is some duplication. As for the commits, I typically squash before merge anyway. I can do that sooner so that the extra cruft isn't around.

[adamlamar]

@kelson42 After looking into the Language filter issue I discovered that it was not constructing the QVariant correctly, even in Qt5. If I understand the intent, the default on first startup is to show you a filtered set of zim files that match your current system language+locale. For example mine is English|en. When printed to the console, the correct language list looks like this:

langList (QVariant(QString, "English|en"))

Two things:

• In Qt5, the language list was always empty, despite the default being set, which made the filter always empty (maybe this feature was not working as intended)
• In Qt6, every character became is its own item in an array of QChar:

langList QList(QVariant(QChar, 'E'), QVariant(QChar, 'n'), QVariant(QChar, 'g'), QVariant(QChar, 'l'), QVariant(QChar, 'i'), QVariant(QChar, 's'), QVariant(QChar, 'h'), QVariant(QChar, '|'), QVariant(QChar, 'e'), QVariant(QChar, 'n'))

After my fixes, the language filter is shown on first start in both:

@veloman-yunkan This should be ready for another review.

[veloman-yunkan]

Having all your changes squashed into a single commit hurts more than it helps. Seeing issues described and resolved one (or a few) at a time is useful.

Please address the new comments as fixup commits so that I don't have to look through the full list of changes during the next (hopefully, last) iteration.

[veloman-yunkan]

LGTM

[kelson42]

@adamlamar Thank you very much for this PR. This is a very nice step forward. We might a few adjustments to do post PR, and might then contact you then.