Better workflow for opening multiple ZIMs in different instances

[Jaifroid]

Currently the workflow for opening a new instance of the PWA is not very intuitive. It involves right-clicking the app's icon, and clicking one of the menu items. If the user has opened a number of recent ZIMs by double-clicking on the ZIM itself, then some ZIMs may show up in this menu and it's possible to choose one from there, which will open a new instance with just that ZIM (see screenshot) at least on Windows.

The problem is that this has some bugs:

1. The selected ZIM gets opened twice and the user is asked both times to answer the security dialogue. That's clearly a bug.
2. Opening a ZIM this way (or by double-clicking the ZIM itself in file explorer) correctly opens in a new window with that ZIM, and other opened windows with different ZIMs still work fine. However, on exiting all windows and re-launching, the list of ZIMs only shows the last opened ZIM, even if that ZIM is from a previously opened folder, forcing the user to re-pick their ZIM directory, even though the app already has access to it.

What's needed is a re-working that allows users to open a new window from the existing in-app list of ZIMs in a folder if they wish. As part of this the file-opening bug need to be fixed, and the app should recognize if a ZIM opened from the file system is in fact in any multi-ZIM folder already open.

Finally, if the OPFS is enabled, opening a ZIM from the FS or (if technically possible) from drag-and-drop, should offer to store the ZIM in the OPFS, or else if the user declines, open it directly.