Create branches for Godot versions

[BastiaanOlij]

I suggest we start creating branches to support different versions of Godot.
Say the current master is branched as "3.0.4" and master is brought up to the current 3.1 master.
That way we can keep master up to date with any changes applied to the Godot master repo.
If 3.0.5 or later get any changes to the header we can simply branch the 3.0.4 branch. Obviously we should only create branches for official godot release that actually have changes in the headers.

Especially for those of us who use submodule this works brilliantly because the repos are linked to specific commits so you can keep branches of GDNative modules for 3.0.x linked to the 3.0.x branch and the latest version linked to whatever commit on master that relates to your godot build.

I also suggest doing the same with godot-cpp

[BastiaanOlij]

I've created a 3.0 branch on an earlier commit, looks like various changes were already added to master that came out of the 3.1 branch. I'm cloning Godot 3.0 atm so I can do a comparison and see if there are any other changes required

I'll also update the readme over the weekend to explain the need for branches.

[BastiaanOlij]

Once #27 and #28 are merged I think we have a stable split where none of the 3.1 changes are in the 3.0 branch and we have an up to date master.