Add ways to list possible apps

[applejag]

I always forget the ID of the launcher or the multiplayer connector app.

Would be nice to mention the system apps in the --help text, and also a way to list all installed games.

For example:

$ firefly-emulator --help
Desktop emulator for Firefly Zero

Usage: firefly-emulator [OPTIONS]

Examples:
  ff import sys.launcher
  firefly-emulator --id sys.launcher

Options:
  ...

And:

$ firefly-emulator --list
lux.snek                  | Snek by Lux: Snake-inspired game with 360° movement.
sys.input-test            | Input test by System: Test touch pad and buttons
sys.launcher              | Launcher by System: The default system launcher