[ENH] DomainEditor: Add horizontal header

[janezd]

Issue

DomainEditor (e.g. in the File Widget) does not have horizontal header and hence doesn't allow the user to resize the columns. This is a problem when the features have longer names.

Description of changes

I added the header. While it is better in terms of functionality, the previous design may have been less cluttered. @BlazZupan, what do you think?

Includes

• Code changes

[codecov-io]

Codecov Report

Merging #2579 into master will increase coverage by <.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #2579      +/-   ##
==========================================
+ Coverage   75.02%   75.03%   +<.01%     
==========================================
  Files         327      327              
  Lines       57641    57646       +5     
==========================================
+ Hits        43248    43252       +4     
- Misses      14393    14394       +1

[ajdapretnar]

Oh yes, I hated this, too!

[janezd]

Codecov/patch failed since tests don't seem to go into return Qt.AlignLeft (although the view in fact uses it) in

if role == Qt.TextAlignmentRole:
    return Qt.AlignLeft

I can obviously write a test that covers this line, but ... won't since there's nothing to test here.

[ajdapretnar]

This caused other problems. Now, it is impossible to resize the widget, even when all data are continuous (Values field is empty). Could you give it a look?

[janezd]

I can't replicate this -- I can resize it.

[ajdapretnar]

Updated the master and now I can, too. Sorry for the panic. :O