shell: Use G_DECLARE_*_TYPE macros

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/shell-app-system.c

@@ -37,6 +37,15 @@ enum {
 
 static guint signals[LAST_SIGNAL] = { 0 };
 
+typedef struct _ShellAppSystemPrivate ShellAppSystemPrivate;
+
+struct _ShellAppSystem
+{
+  GObject parent;
+
+  ShellAppSystemPrivate *priv;
+};
+
 struct _ShellAppSystemPrivate {
   GHashTable *running_apps;
   GHashTable *id_to_app;