Allow Quit from Initial Setup

[elsiehupp]

Fixes #2969.

I've tested this on macOS but not on Linux or Windows.

I'm thinking of adding my code from #2959 that shows and hides the dock icon, as well. (I'll put it in a separate commit so it's easier to test.)

[elsiehupp]

I went ahead and incorporated the code from #2959. FWIW the code could probably be generalized using the QWinTaskbarButton Class to show and hide the taskbar icon on Windows similarly to how the code currently works on macOS. I don't have a Windows install set up currently, though, so I wouldn't be able to implement and test it in the immediate future. I don't know what's involved in showing and hiding the dock icon on Linus.

[elsiehupp]

Looking more closely at my screenshots, the "Back" button on the one wizard page is for some reason not aligned with the "Next" and "Cancel" buttons:

[elsiehupp]

I implemented changes so that reject() will only quit the application if there are no accounts set up. I also tried to implement code that would make it so that the "Cancel" button only appears on the first page, but now the "Cancel" Button doesn't seem to be appearing at all.

[elsiehupp]

I also noticed the horrendous kerning in the main title:

In particular see the "z" overlapping with the "a" and the huge gap between the "Q" and the "u". What font is this?!?

(And I just noticed the word "Setup" [noun] instead of the phrase "Set up" [verb].)

[er-vin]

Nice, much more tidy and focused. Much less confusing to review, thanks!

Will probably need grooming the history in that branch though, other than that it looks like it's getting there.

[nextcloud-desktop-bot]

AppImage file: Nextcloud-PR-2974-3b0f663612d0d932f52c04fc447c62344e7e62be-x86_64.AppImage

To test this change/fix you can simply download above AppImage file and test it.

Please make sure to quit your existing Nextcloud app and backup your data.