st: Use G_DECLARE_*_TYPE

Cut down on boilerplate by using the (no longer that) new helper
macros. We don't care about breaking ABI in private libraries, so
use G_DECLARE_FINAL_TYPE even where the class struct used to be
exposed in the header, except for types we inherit from ourselves
(obviously) or where the class exposes any vfuncs (where changes
could affect inheritance in extensions).

src/st/st-theme-node-private.h

@@ -116,11 +116,6 @@ struct _StThemeNode {
   StThemeNodePaintState cached_state;
 };
 
-struct _StThemeNodeClass {
-  GObjectClass parent_class;
-
-};
-
 void _st_theme_node_ensure_background (StThemeNode *node);
 void _st_theme_node_ensure_geometry (StThemeNode *node);
 void _st_theme_node_apply_margins (StThemeNode *node,