Use a popup, like we do for properties. For now, it allows activating the action (with parameter, if necessary) and setting the state.
React to action group changes, and allow changing action states.
Show the actions that are added to GtkApplication and GtkApplicationWindows, as well as action groups that are inserted elsewhere with gtk_widget_insert_action_group. https://bugzilla.gnome.org/show_bug.cgi?id=730095
