pmb.config.init: prompt for installing UI extras package

This check if the selected ui has an extras subpackage and ask the user about installing it.

I wasn't sure about which subpackage name to use, so I followed the existing postmarketos-ui-plasma-mobile-extras.

Please let me know (here or in #1796 (closed)) if we want extras or something else.

TODO: probably fix some tests

Closes #1796 (closed)