FEAT(GUI): Hierarchical column header view

[tmichela]

Adds hierarchical columns header view, to improve visually grouping Variables that belong together.

Hierarchy is built from variable's title: each '/' character splits the title to the next level.
Will be more important once #478 is merged.

[codecov]

Codecov Report

❌ Patch coverage is 84.90566% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.10%. Comparing base (93a2b3d) to head (bf5c321).

Files with missing lines	Patch %	Lines
damnit/gui/table.py	84.90%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
+ Coverage   78.84%   79.10%   +0.25%     
==========================================
  Files          37       37              
  Lines        6562     6771     +209     
==========================================
+ Hits         5174     5356     +182     
- Misses       1388     1415      +27     

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[takluyver]

The screenshot looks pretty nice. 👍 Do we want to impose some kind of sanity limit on the hierarchy depth? Or just trust people to be sensible with it?

There's quite a bit of detailed Qt code here using APIs that I'm not familiar with. Do you want to do a pair review in the new year? Or I'm happy to trust you on it, but maybe if we do that we should make an off switch to quickly go back to simple table headings in case of bugs (and then make a note to get rid of it in a few more months).