ui: add preference for applicationShouldTerminateAfterLastWindowClosed

[ktprograms]

I find it quite convenient that UTM automatically quits after I shut down and close my VM.

[conath]

I wonder why this is not the default setting right now. It makes sense to me that if you closed the UTM main window and then close the last VM, UTM would quit.

[ktprograms]

It used to be the default, but in 23e03a2 it was changed to not automatically quit, in response to #3480

[osy]

I wonder why this is not the default setting right now. It makes sense to me that if you closed the UTM main window and then close the last VM, UTM would quit.

Just can't satisfy everyone smh