all: bump Gtk+ version to 3.10

We've accidentally depended on 3.10 features for some time (widget templates,
revealers) and 3.10 is already rather old. (Even RHEL 7 has a newer one).

Also bump GLib to 2.38. The Gtk 3.10 requires it and we already use
features from versions newer by 2.32 (e.g. g_object_replace_data()).