Bump version to 3.0-stable \o/

Congratulations to everyone in the Godot community for the tremendous work
done on this release since 18 months, with hundreds of contributors pushing
almost 7500 commits with more than 3000 PRs and closing over 2000 issues
(and fixing even more than that, as many work-in-progress bugs were fixed
before an issue could be filled).

Godot 3.0 is definitely our biggest and boldest release so far, and we want
to thank the whole community for their unswerving support during this long
wait.

From there on, there is a lot of work to do to strengthen the foundations
that we built with 3.0, fixing the bugs that the many refactorings probably
introduced, optimizing new features and enhancing the usability again...

The 3.x era should be a fruitful one for Godot, and we hope that you will
continue using it to create awesome 2D and 3D games and increase the
notoriety of your favourite engine in the game development industry.

And now, let's all start waiting for 3.1...

version.py

@@ -2,5 +2,5 @@
 name = "Godot Engine"
 major = 3
 minor = 0
-status = "rc3"
+status = "stable"
 module_config = ""