Improved UI for new main screens

This adds some bells and whistles to the UI in !433 (merged), and also makes the app more compatible with the current stable build:

  • General improvements to UI (e.g. app lists).
  • Improved accessibility.
  • Bug fixes.
  • Search button.
  • Handle all the intents that the old FDroid class used to.

As with !433 (merged), it is a large change but it should build and run after essentially every commit. That way, if you get bored of CR'ing half way through you can stop and we can consider merging up to there after addressing any feedback.

(Fixes #734, Fixes #867 (closed))