Add GameMode support

[kenvandine]

There are many resources out there that suggest users add gamemoderun to the launch command, enabling linux GameMode. We should stage that package and set $PATH appropriately to allow this to work when specified.

Staging it is trivial, but the question is what happens when run from within confinement, we may need some interface work to allow it to talk to the daemon on the host. Let's evaluate that and propose it to snapd, as needed.

[ashuntu]

This is basically done, just waiting on this PR flatpak/xdg-desktop-portal#882 and for the GameMode upstream change to be released in an official version for it to be usable.