File Loading Dialogue

[Julian-Hochhaus]

Fix Issues with the file-loading dialogue:

1. If Cancel is pressed, the Program crashes; instead, if Cancel is pressed, no plotting/ no further work should be done by the program
2. remember_settings might have some bugs, and not all settings are directly applied
3. Implement a fallback that always checks if the file that should be imported is formatted like the files exported by the program and then skips the preview dialogue.