GtkApplication: Fix two muxer-related memory leaks
GtkApplication: Fix two muxer-related memory leaks

Drop the ref on the action muxer in finalize, and also make sure
shutdown() tears down the muxer setup done in startup().

When GtkApplication adds itself to a muxer, it causes the muxer to take
a ref on the GtkApplication. This has to be undone in shutdown() to make
sure the GtkApplication doesn't end up holding a ref on itself.

https://bugzilla.gnome.org/show_bug.cgi?id=730383

Lyude/gtk-,davidgumberg/gtk,Lyude/gtk-,chergert/gtk,ahodesuka/gtk,alexlarsson/gtk,chergert/gtk,davidgumberg/gtk,jessevdk/gtk,ahodesuka/gtk,ahodesuka/gtk,Adamovskiy/gtk,Lyude/gtk-,Sidnioulz/SandboxGtk,jigpu/gtk,jadahl/gtk,Lyude/gtk-,jigpu/gtk,grubersjoe/adwaita,grubersjoe/adwaita,chergert/gtk,chergert/gtk,jigpu/gtk,ahodesuka/gtk,Lyude/gtk-,Sidnioulz/SandboxGtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,Adamovskiy/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jadahl/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,jigpu/gtk,jadahl/gtk,alexlarsson/gtk,ahodesuka/gtk,jessevdk/gtk,grubersjoe/adwaita,chergert/gtk,alexlarsson/gtk,jadahl/gtk,ahodesuka/gtk,ahodesuka/gtk,Adamovskiy/gtk,alexlarsson/gtk,chergert/gtk,Lyude/gtk-,Lyude/gtk-,davidgumberg/gtk,grubersjoe/adwaita,Adamovskiy/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk,Adamovskiy/gtk,Adamovskiy/gtk,ahodesuka/gtk,jessevdk/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,davidgumberg/gtk,davidgumberg/gtk,davidgumberg/gtk,jessevdk/gtk,jigpu/gtk,jigpu/gtk,grubersjoe/adwaita,jessevdk/gtk,jadahl/gtk,jessevdk/gtk,jigpu/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,jadahl/gtk,chergert/gtk,chergert/gtk,davidgumberg/gtk,Lyude/gtk-,davidgumberg/gtk