Release from the version in project.godot

The version now lives in project.godot (application/config/version)
and Settings.version reads it from there. The Build workflow tests and
exports on every push, and on main creates the tag, uploads the builds
and creates the Gitea release when that version has no tag yet. This
replaces gitea-release.yaml, which released when a branch named after
the version was pushed.

Godot setup moves into a local .gitea/actions/setup-godot action shared
by the test and build jobs, and Godot is updated to 4.7.2 to match the
editor the project is made with.

RELEASING.md

@@ -0,0 +1,20 @@
+# Releasing
+
+Releases are made from the version in `project.godot`
+(**Project > Project Settings > Application > Config > Version**). The game shows the
+same version in the About window.
+
+1. Bump the version, e.g. from `1.9.0` to `1.10.0`, and add a `WHATS_NEW` entry in
+   `AboutWindow.gd`.
+2. Merge to `main` and push.
+
+The `Build` workflow (`.gitea/workflows/gitea-build.yaml`) then:
+
+1. runs the tests,
+2. exports the Windows and Linux builds,
+3. if no tag exists for the version yet: uploads the builds to the package registry
+   under `MusicPlayer/<version>/`, and creates the `<version>` tag and Gitea release
+   with the builds attached.
+
+Pushes that don't change the version, and pushes to `develop`, only test and export.
+Don't create tags or `1.x.y` branches by hand; the workflow makes the tag.