Added mnemonics to main window menu

[amibranch]

Your checklist for this pull request#3518

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've updated the documentation with the relevant information (if needed)

Detailed description

The change adds Mnemonics to the main window menu

Test plan (required)

Tests for languages could not be performed locally. Any feedback on how to deal or even test with translations is welcomed!

Closing issues

closes #3518

[karliss]

If you are going to do this might at as well do it a bit thoroughly not just the top level menu labels. Not necessarily every item in every submenu, but a good chunk of them could use it.

[amibranch]

As suggested, I added mnemonics to some sub-menu points. Also, I added mnemonics to the main buttons in the "NewFileDialog"-Window. If you have any other dialogs in mind where you feel that adding mnemonics provides a benefit, give me a note.

However, just a brief check of the translations revealed that they seem to be not working for any string where mnemonics have been added. Do you prefer one of the options:

• Adding a new translation for the strings with added mnemonics (by doing so, it would still be compatible with versions without mnemonics)
• Replacing the strings in the translations?
  Or should I raise this question directly in cutter-translations?

Note: I guess this PR is blocked now by adding mnemonics-translations to cutter-translations first? Or is this done automagically by CI?