Tray icon issues on macOS 10.13.5

[szszszsz]

• [ ] the tray icon was not visible on black theme on macOS 10.13.5 - Nitrokey App v1.3 do have the tray icon coloring, but only for the Linux/Windows AFAIR (due to some potential compatibility issues);
• [x] tray icon's Quit action is not working - to investigate;
• [ ] tray icon's Quit action is not working - follow advices from issue 343

[oderwat]

Tray Icon quit action problems were reported by me in section 3 of #343 (10 weeks ago), which also goes into details about the reasons.

[szszszsz]

Well, that closes the investigation, thank you! :-) Hopefully I will take after it next month. Fix for this one should be straightforward.

[muellermartin]

The tray icon is still not visible on macOS in dark mode:

This is problematic, as this is the only way to access the 'Configure' menu in the English UI translation (see issue #366).

[szszszsz]

Bumping