Add Windows→Hide docks menu item. We don't remove the hardcoding of
having Tab trigger it though, because gtk_accelerator_valid() returns
FALSE for GDK_tab. This means that if the user changes keyboard
shortcut for the menu item, both the user shortcut and the hardcoded
shortcut will work. We remove gimp_dialog_factories_toggle() and make
clients activate the action instead to toggle dock visibility.