Fixing a few issues on the Steam Deck (KDE environement)

[beanbag420]

• The icon doesn't work correctly in KDE (both the window icon and taskbar icon). They both show the xterm icon.
• The desktop file needs to be completely redone
• Metadata file is missing and is probably causing some issues, I'll need it anyway for a flathub submission
• Adding REDRIVER2 as a non-steam game on SteamOS (non-desktop mode) crashes after "Play the game" is clicked. (This might be because of missing env variables)
• I think having two options or even two desktop files could help the situation on the deck (ex: Start with the launcher and start the game)