Change menu action on macOS

[2V3EvG4LMJFdRe]

Describe the solution you'd like

I want to request the ability to change the "Prefer dark mode" setting from the View menu, as most programs allowing for a theme change do. I'd use this to be able to control the setting with an Automator script and create a day/night theme schedule.

I also want to point out that the items placed under File should probably go under the main Nicotine+ entry per standards of macOS.

Additional context

Some images for comparison:

[mathiascode]

I've added a Command+, shortcut to avoid disabling the 'Preferences' menu item, but I'm not going to complicate the menu logic further.

[2V3EvG4LMJFdRe]

As an alternative to the first request, could you add a way to tell Nicotine to "Prefer dark mode" from the terminal?

[mathiascode]

The first request was implemented in this commit. :)
d63cda5