[RFC] Create postmarketos-base-ui-gnome to store common configurations between gnome and phosh

There already exists some redundancy in schemas and dependencies between postmarketos-ui-phosh and postmarketos-ui-gnome which could be removed by creating a new base package. Moreover, there is some functionality that is only packaged in one of them, but for which the other could also benefit, e.g: 02-gnome-software-no-first-run in the phosh package.