Bump version to 3.0-beta a.k.a. *feature freeze*

*Feature freeze* means that from now on, no new features will be considered
for Godot 3.0, unless explicitely decided by core developers.

New pull requests implementing additional features will be automatically
set for the 3.1 milestone, and will only be considered for merging once the
3.0 version goes stable and the *master* branch reopens for feature
development.

Existing PRs made before the freeze will still be reviewed and potentially
be merged, if the features that they implement are deemed important enough
or don't risk to introduce issues.

Otherwise, PRs should now focus on:
- Fixing bugs
- Enhancing existing 3.0 features

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