[ENH] Create corpus - new widget

[PrimozGodec]

Issue

Sometimes it would be handy to create corpus with just pasting few documents

Description of changes

Create a corpus widget that enables create a simple corpus by pasting or typing documents directly in Orange

Todo

• Support removing documents
• tests
• few details marked with todo in the widget

Includes

• Code changes
• Tests
• Documentation - will be added when we agree that a widget is in the right shape

[codecov-commenter]

Codecov Report

Merging #854 (bb6bd21) into master (4c38b3f) will increase coverage by 0.16%.
The diff coverage is 98.05%.

❗ Current head bb6bd21 differs from pull request most recent head 32c1629. Consider uploading reports for the commit 32c1629 to get more accurate results

@@            Coverage Diff             @@
##           master     #854      +/-   ##
==========================================
+ Coverage   76.67%   76.84%   +0.16%     
==========================================
  Files          83       84       +1     
  Lines       11761    11873     +112     
  Branches     1867     1875       +8     
==========================================
+ Hits         9018     9124     +106     
- Misses       2439     2444       +5     
- Partials      304      305       +1     

[ajdapretnar]

The widget looks good. I would perhaps make the initial boxes even shorter (less tall), but I don't object to the current situation.

[PrimozGodec]

The widget looks good. I would perhaps make the initial boxes even shorter (less tall), but I don't object to the current situation.

I just changed the default to 3 documents. Now boxes are a bit shorter by default

[ajdapretnar]

@PrimozGodec I approve the widget. I added the documentation, too.

[PrimozGodec]

@ajdapretnar thanks a lot for help with the documentation. :)