set StartupWMClass in linux `.desktop` file
this allows linux app launchers and docks to correctly associate the godot project selector & editor windows with the icon used to launch godot.
it prevents this problem specifically: https://askubuntu.com/questions/1144214/why-does-my-custom-launcher-file-create-two-icons-on-the-launcher-bar
(cherry picked from commit 7b346258a991d205fb203034af99c8cf80b301ee)
backwardspy