Add txt file filter for saving output file

[andreasmarnold]

Closes #89, preventing images from being overwritten by the affine parameters, by:

• changing the label of the file selection box to make it clearer that it's for saving affine parameters, and
• adding a file filter to the file selection widget, so it only shows .txt files.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6cb4d95) 93.84% compared to head (859eff6) 93.84%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #90   +/-   ##
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  Coverage   93.84%   93.84%           
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  Files           7        7           
  Lines         276      276           
=======================================
  Hits          259      259           
  Misses         17       17           

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