main: Use gtk_actionable_set_action_name()

Rather than setting the property by hand.
main: Move the "un-fullscreen" button to the corner

So that it's easier to reach when in fullscreen mode.

Requires:
https://git.gnome.org/browse/gnome-themes-standard/commit/themes/Adwaita/gtk-3.0?id=0f705f2097e44778eb665782f2cf23a889b2c73c

https://bugzilla.gnome.org/show_bug.cgi?id=727224