Cleanups: fix deprecated g_type_class_add_private()

- use G_DECLARE_ and G_DEFINE_ macros
- move all data into ClassNamePrivate
- use _get_instance_private()

This should not introduce any functional changes or breakage.

Skipped two classes (EekKeyboard and EekboardContextService) for now in
order not to break the build.

These two classes are used in some very funky WIP code that tries to
circumvent encapsulation.

(Funky code is in eekboard/key-emitter.c and eekboard/eekboard-context-service.c)

eekboard/eekboard-client.h

@@ -26,22 +26,7 @@
 G_BEGIN_DECLS
 
 #define EEKBOARD_TYPE_CLIENT (eekboard_client_get_type())
-#define EEKBOARD_CLIENT(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), EEKBOARD_TYPE_CLIENT, EekboardClient))
-#define EEKBOARD_CLIENT_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), EEKBOARD_TYPE_CLIENT, EekboardClientClass))
-#define EEKBOARD_IS_CLIENT(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), EEKBOARD_TYPE_CLIENT))
-#define EEKBOARD_IS_CLIENT_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), EEKBOARD_TYPE_CLIENT))
-#define EEKBOARD_CLIENT_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), EEKBOARD_TYPE_CLIENT, EekboardClientClass))
-
-typedef struct _EekboardClient EekboardClient;
-typedef struct _EekboardClientClass EekboardClientClass;
-typedef struct _EekboardClientPrivate EekboardClientPrivate;
-
-struct _EekboardClient {
-    /*< private >*/
-    GDBusProxy parent;
-
-    EekboardClientPrivate *priv;
-};
+G_DECLARE_DERIVABLE_TYPE (EekboardClient, eekboard_client, EEKBOARD, CLIENT, GDBusProxy)
 
 struct _EekboardClientClass {
     /*< private >*/