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 :white_check_mark:

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

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Additional details and impacted files

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

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