Allow installing desktop files while installed as native

Plans had been made that we should release a build without flatpak's sandbox. Currently doing this will work fine for just playing the game, but for modding, this will become an issue due to the lack of a URI scheme being installed to the system.

The solution should be as simple as following these steps:

• Embed or generate a .desktop
  • Can use what we have for flatpak as a base
  • Change Exec to the location of the game's executable binary.
  • Change Icon to UnleashedRecomp
• Install desktop file and icon
  • Desktop files can be installed/linked to ~/.local/share/applications with a name like UnleashedRecomp.desktop
  • Icons files can be installed/linked to .local/share/icons/hicolor/128x128/apps with a name like UnleashedRecomp.png (Make sure to create the folder if missing)
  • Update applications cache by running update-desktop-database ~/.local/share/applications
• Register the mime type
  • Just run xdg-mime default UnleashedRecomp.desktop x-scheme-handler/unleashedrecomp

For most Linux systems this should work without any issues.

Start by locating the existing Flatpak .desktop definition and the native installation entry point. Use the Flatpak file as the base, adjust the executable and icon references, and verify that the desktop file, icon, cache update, and unleashedrecomp URI handler are installed under the stated Linux paths.