Improve tool tips

[soininen]

Use rich text in tool tips so Qt can wrap the text.

Re spine-tools/Spine-Toolbox#2542

Checklist before merging

• Code has been formatted by black
• Unit tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 60.85%. Comparing base (f1c61c6) to head (4228d6e).

Files	Patch %	Lines
...s/exporter/mvcmodels/mapping_editor_table_model.py	33.33%	2 Missing ⚠️
spine_items/importer/mvcmodels/mappings_model.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           0.8-dev     #193      +/-   ##
===========================================
+ Coverage    60.84%   60.85%   +0.01%     
===========================================
  Files          193      193              
  Lines        17879    17877       -2     
  Branches      2892     2891       -1     
===========================================
+ Hits         10878    10879       +1     
+ Misses        6415     6414       -1     
+ Partials       586      584       -2     

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