Ensure G_PARAM_STATIC_STRINGS for all properties

In `st`, we can do this by using `ST_PARAM_*`. In the other code files,
just use `G_PARAM_STATIC_STRINGS` directly.

This is just a minor convenience to prevent a few unnecessary string
copies.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/646

src/shell-window-tracker.c

@@ -103,7 +103,7 @@ shell_window_tracker_class_init (ShellWindowTrackerClass *klass)
                                                         "Focus App",
                                                         "Focused application",
                                                         SHELL_TYPE_APP,
-                                                        G_PARAM_READABLE));
+                                                        G_PARAM_READABLE | G_PARAM_STATIC_STRINGS));
 
   signals[STARTUP_SEQUENCE_CHANGED] = g_signal_new ("startup-sequence-changed",
                                    SHELL_TYPE_WINDOW_TRACKER,