[FIX] owselectcolumns: Fix move up/down type error

[ales-erjavec]

Issue

Selecting a variable in the "Features" list and pressing Up/Down button (to the left of the view) triggers a:

Traceback (most recent call last):
  File "/Applications/Orange3.app/Contents/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/Orange/widgets/data/owselectcolumns.py", line 436, in move_down
    self.move_rows(view, selected, 1)
  File "/Applications/Orange3.app/Contents/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/Orange/widgets/data/owselectcolumns.py", line 416, in move_rows
    newrows = [min(max(0, row + offset), len(model) - 1) for row in rows]
  File "/Applications/Orange3.app/Contents/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/Orange/widgets/data/owselectcolumns.py", line 416, in <listcomp>
    newrows = [min(max(0, row + offset), len(model) - 1) for row in rows]
TypeError: object of type 'VariableFilterProxyModel' has no len()

Description of changes

Use QAbstractItemModel API to exchange the variables..

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4271 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4271      +/-   ##
==========================================
+ Coverage   86.22%   86.24%   +0.02%     
==========================================
  Files         396      396              
  Lines       70706    70726      +20     
==========================================
+ Hits        60964    61000      +36     
+ Misses       9742     9726      -16