Remove dead orangeqt code; add gui.listView multiselection handling

[kernc]

Description of changes

Some destructive changes split from #2327 and some other changes required by biolab/orange3-prototypes#99.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #2761 into master will increase coverage by 4.53%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2761      +/-   ##
==========================================
+ Coverage    76.2%   80.74%   +4.53%     
==========================================
  Files         337      324      -13     
  Lines       59665    56227    -3438     
==========================================
- Hits        45469    45400      -69     
+ Misses      14196    10827    -3369

[astaric]

Hooray for removals.

@janezd, could you check the changes to Call[Back|Front]ListView?

[janezd]

I think it's better to select all items at once, which emits fewer signals -- and also simplifies the code. I also added some tests. See c30d658.

Hooray for removals, too.

[janezd]

I think it's better to select all items at once, which emits fewer signals -- and also simplifies the code. I also added some tests. See c30d658.

Hooray for removals, too.

[janezd]

Pylint results
=================================================================
Number of issues on parent commit: 1188
Number of issues on the pull request: 30 (-1158)

I suppose this must be a record.