Add file type filters to QFileDialogs based on selected tool type

[ptsavol]

Fixes spine-tools/Spine-Toolbox#2961

Checklist before merging

• Documentation (also in Toolbox repo) is up-to-date
• Release notes in Toolbox repo have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.

Project coverage is 63.02%. Comparing base (53e29bd) to head (6bb6b42).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...s/tool/widgets/tool_specification_editor_window.py	53.84%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
- Coverage   63.03%   63.02%   -0.02%     
==========================================
  Files         168      168              
  Lines       17959    17970      +11     
  Branches     2066     2069       +3     
==========================================
+ Hits        11320    11325       +5     
- Misses       6038     6042       +4     
- Partials      601      603       +2     

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