meson: Bump minimal GLib version to 2.44

This means we can use some newer features and get rid of some
boilerplate code using the G_DECLARE_* macros.

As discussed on IRC, 2.44 is old enough by now to start depending on it.
17 jobs for wip/nielsdg/bump-glib in 100 minutes and 7 seconds (queued for 1 second)
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