Flathub integration
It's about time that we started promoting content from Flathub. My suggestion for this is to:
- Show a "New in Flathub" and/or "Popular from Flathub" section on the front page
- Drop the existing repository identifiers from app tiles and list rows, and just show one entry per app, irrespective of how many install options there are for it
- If there are multiple sources for the same application, show a dialog when "Install" is pressed, to ask where it should be installed from
- Allow filtering search results by source