Improve mangohud and gamescope missing binaries message for Flatpak

[arielj]

Problem description

Currently, when mangohud or gamescope are not found in PATH we should a message saying that, but when using Flatpak it's not clear for the users that the mangohud/gamescope flatpak package is needed, that the system-wide version won't work.

Feature description

Since we can know when we are running as a Flatpak, we can improve those message and mention the name of the needed package.

We should also try to mention which flatpak runtime version they need to install, cause installing gamescope 23 wouldn't work if heroic uses 24 for example.

Alternatives

No response

Additional information

I don't know if there's any other similar binary/wrapper that works the same way, but we should do the same in those cases.