build: Use subdir-objects
actor: Queue a relayout if we use the content's preferred size

In case the ClutterContent changes, and the actor uses the content's
preferred size to drive its own.
docs: Mention ClutterRequestMode in the release notes
Bump up the requirement for GLib and introspection

The syntax for some introspection annotations has changed between 1.38
and 1.39, so we need to bump up the dependency in order to get the new
scanner. Introspection should be updated in lock-step with GLib, so we
should also bump up the required GLib version.
Use new introspection annotation syntax

Instead of the deprecated one.
build: Fix out-of-tree builds

Add a srcdir prefix to the inspected files for glib-mkenums.
build: More out of tree build fixes
Release Clutter 1.21.2
Post-release version bump to 1.21.3