[ENH] OWPythonScript: dropping and pasting of python scripts

[markotoplak]

Issue

Drag and drop of files did not work in the Python script widget

Description of changes

Files can be dragged and dropped (or pasted) onto the script editor or widget's control area. Nothing changed for dropping or pasting text.

Includes

• Code changes
• Tests
• Documentation

[markotoplak]

As @BlazZupan suggested, now, this widgets pop ups a dialog that asks whether to replace the current script or append to it.

Another option (that I prefer) is to never ask and always insert the text at the cursor position.

[codecov]

Codecov Report

Merging #3611 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3611      +/-   ##
==========================================
+ Coverage   84.24%   84.26%   +0.01%     
==========================================
  Files         370      370              
  Lines       67475    67565      +90     
==========================================
+ Hits        56844    56932      +88     
- Misses      10631    10633       +2

[markotoplak]

We talked about it with @BlazZupan and we decided that if a user drags/pastes in a file, the current script is replaced with that file (there are no pop-up dialogs asking you where to put it as in the first attempt). I took care that undo still works.