Flatpak Support

[SeongGino]

I've never actually looked at or done anything with Flatpak outside of using some (select few) apps through it, so I'm not sure what the requirements of Flatpak support are.

FWIW: regarding filesystems, we rely currently on a couple of things that are found in the local filesystem:

• umu-run itself, because duh
• curl for external downloading (currently only used for downloading the Discord Bridge service, but will be used elsewhere), waiting on another issue to pack this in as a build dependency rather than runtime.
• ~/.steam/steam/compatibilitytools.d for finding existing Steam Proton runners - not completely necessary, as Nero Manager can simply make its own directory that it uses to store runners.
• ~/.config for the manager configuration file, which stores the prefixes path and basic settings for the frontend.
• Whatever gets defined as the prefixes "Home"
• /tmp, for storing either downloads (currently only Discord Bridge) or icons extracted from applications.

Nero Manager also provides a system tray icon, but I assume Flats should be able to use these just fine?

I have to assume, due to sandboxing, that any access to filesystems needs to go through some kind of filter or some shit?