Support -extra subpackage in UI meta packages

Currently we have a postmarketos-ui-plasma-mobile-extra package which depends on postmarketos-ui-plasma-mobile and some extra useful Plasma applications. This is however a separate APKBUILD entirely, and appears as a new entry in the UI list of pmbootstrap init.

It would be nice to support an $pkgname-extra or $pkgname-apps (upstream Alpine uses the latter with the gnome meta package) subpackage, which appears as a new question in pmbootstrap init. Something like, "The UI you have chosen has some additional apps available, do you want to install them?". If the answer of the user is yes, the subpackage will be installed. This way removing the subpackage from a device also won't cause the UI meta package to be uninstalled, as the meta package is installed explicitely by pmbootstrap and isn't just a dependency of the -extra package.