Separate Godot Export Manager from the Collada exporter

[CowThing]

Currently the Godot Export Manager only supports exporting Collada files. But since glTF 2.0 and Godot's exported scene are going to be more popular exports in the future, I think the Godot Export Manager should be separated from Better Collada and be made to support all 3 exports, allowing the user to select which export to use.