Some groundwork for the main screens new UI

Merged username-removed-25042 requested to merge pserwylo/fdroidclient:preperation-for-new-ui into master

Breaking up in the interest of making it easier to request CRs.

This includes a few small commits laying the groundwork for the new UI. The commits are separated out from the other stuff on the basis that they can be merged in without (drastically) changing any behaviour of the current F-Droid client. The only exception is the addition of the preference "Manage Repositories" which will be available both in the main menu and also the settings activity (In the forthcoming UI changes, the main menu will not exist).