[ENH] Create nodes on canvas drag/drop

[ales-erjavec]

Issue

Closes #4918

Description of changes

Uses:

• [ENH] Plugin drag/drop support orange-canvas-core#143
• [ENH] Add canvas drag/drop utility classes orange-widget-base#106

Implements drop handlers for 'File', 'Load Model', 'Distance File' and 'Python Script' widgets.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #5031 (e8006ec) into master (c2b8582) will decrease coverage by 0.00%.
The diff coverage is 96.22%.

@@            Coverage Diff             @@
##           master    #5031      +/-   ##
==========================================
- Coverage   86.34%   86.34%   -0.01%     
==========================================
  Files         304      304              
  Lines       61619    61667      +48     
==========================================
+ Hits        53206    53247      +41     
- Misses       8413     8420       +7     

[irgolic]

This is really cool, thanks. Changes like this go a long way, as minor as they may seem on the surface.

How hard would it be to support drag-and-dropping several files at a time to canvas?

As is right now though, most any file I try drag-and-dropping (.csv, .tab, .dst, .py) gives the option of using CSV File Import (alongside another). For .py, .dst, this shouldn't happen.
The point of this change is an intuitive way to load your data into Orange. But by dragging a file (.csv, .tab...) to canvas, a dropdown is (correctly) shown offering either File or CSV File Import – something totally confusing for a new Orange user, REALLY underscoring that #3997 needs to be fixed. I'd even wait with merging this before that's fixed.

Also, a nitpick, it gets laggy when holding a file over the window (this PR checked out on left, latest release on right)

Any chance some caching could come into place for this to only lag the first time the file touches the window? (or not lag at all?)