Slimmed down variant from the reverted #8375.
The rationale behind the name change is to give Godot's project file a unique
extension (".godot") that can be registered on the OS to be associated with
the Godot binary (OS registration not implemented here).

This PR also adds the possibility to start the game or editor if launched
with the project.godot passed as argument, which paves the way for allowing
a similar behaviour on a double-click in the OS file manager (code originally
by @Hinsbart).

Closes #6915.