[FIX] listfilter: Fix filter line edit completion

[ales-erjavec]

Issue

Inline filter completion in the Select Columns widget does not work.

Description of changes

Make QCompleter a child of the filter line edit.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #2896 into master will increase coverage by 0.25%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2896      +/-   ##
==========================================
+ Coverage   82.12%   82.37%   +0.25%     
==========================================
  Files         329      335       +6     
  Lines       56258    57955    +1697     
==========================================
+ Hits        46201    47742    +1541     
- Misses      10057    10213     +156

[lanzagar]

This results in the filter line auto-completing to the first hit and the box below shows only that one feature...
E.g. Iris with all attributes on the left; when I write "sepal" I see only one feature instead of two.

If the filter box shows the first hit, I would expect to see all matching variables in the box below, until pressing enter when the auto-complete suggestion is accepted.

[lanzagar]

Any suggestions on how to implement completion so as not to interfere with regular filtering tasks?
@ales-erjavec or someone else?

[astaric]

The completion now only works on variable attributes, so it has no effect on regular filtering.