background-view: Disconnect pre-mapped handler after setting up view

If we don't do this and the view is killed or dies, new views might be matched
and set unintentionally as a background view.

src/background-view.cpp

@@ -362,6 +362,7 @@ class wayfire_background_view : public wf::plugin_interface_t
 
         // Remove any idle inhibitors which were already set
         remove_idle_inhibitors();
+        on_view_pre_map.disconnect();
     }
 
     wf::signal::connection_t<wf::view_pre_map_signal> on_view_pre_map = [=] (wf::view_pre_map_signal *ev)