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/st/st-widget.c

@@ -989,7 +989,7 @@ st_widget_class_init (StWidgetClass *klass)
                                                       "The accessible role of this object",
                                                       ATK_TYPE_ROLE,
                                                       ATK_ROLE_INVALID,
-                                                      G_PARAM_READWRITE));
+                                                      ST_PARAM_READWRITE));
 
 
   /**