Fix issues with Importer's source selector dialog

[soininen]

• If souce type stored in the mapping clashes with source type, user now gets a warning before seeing the source selector.
• In the above case and if user chooses a different source type to what is defined in the mapping, the undo stack is marked as dirty so it is actually possible to save the specification afterwards.
• The source selector dialog is now shown only after the specification window is shown. This enables showing correct icon in the dialog as well.

No associated issue

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 58.82353% with 7 lines in your changes missing coverage. Please review.

Project coverage is 63.01%. Comparing base (49f859a) to head (df47e20).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...ine_items/importer/widgets/import_editor_window.py	58.82%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
- Coverage   63.02%   63.01%   -0.01%     
==========================================
  Files         168      168              
  Lines       17970    17979       +9     
  Branches     2069     2070       +1     
==========================================
+ Hits        11325    11329       +4     
- Misses       6042     6046       +4     
- Partials      603      604       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.