[ENH] Plugin drag/drop support

[ales-erjavec]

Ref: biolab/orange3#4918

Implement plugin support for handling canvas drop events.

[codecov-io]

Codecov Report

Merging #143 (49348e1) into master (8de1f3e) will decrease coverage by 0.32%.
The diff coverage is 84.24%.

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
- Coverage   71.64%   71.32%   -0.33%     
==========================================
  Files          95       95              
  Lines       19450    19736     +286     
==========================================
+ Hits        13935    14076     +141     
- Misses       5515     5660     +145     

Impacted Files	Coverage Δ
orangecanvas/application/canvastooldock.py	62.76% <ø> (-0.27%)	⬇️
orangecanvas/application/widgettoolbox.py	61.29% <ø> (ø)
orangecanvas/application/canvasmain.py	68.88% <23.80%> (-0.81%)	⬇️
orangecanvas/document/interactions.py	41.84% <87.30%> (+9.99%)	⬆️
orangecanvas/canvas/scene.py	80.85% <87.50%> (+0.43%)	⬆️
orangecanvas/document/schemeedit.py	74.23% <87.50%> (+2.04%)	⬆️
orangecanvas/gui/test.py	96.47% <96.77%> (+0.04%)	⬆️
orangecanvas/utils/__init__.py	78.35% <100.00%> (+0.22%)	⬆️
orangecanvas/gui/framelesswindow.py	82.00% <0.00%> (-18.00%)	⬇️
orangecanvas/gui/splashscreen.py	74.69% <0.00%> (-16.87%)	⬇️
... and 16 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8de1f3e...49348e1. Read the comment docs.