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-context.h

@@ -28,28 +28,7 @@
 G_BEGIN_DECLS
 
 #define EEKBOARD_TYPE_CONTEXT (eekboard_context_get_type())
-#define EEKBOARD_CONTEXT(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), EEKBOARD_TYPE_CONTEXT, EekboardContext))
-#define EEKBOARD_CONTEXT_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), EEKBOARD_TYPE_CONTEXT, EekboardContextClass))
-#define EEKBOARD_IS_CONTEXT(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), EEKBOARD_TYPE_CONTEXT))
-#define EEKBOARD_IS_CONTEXT_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), EEKBOARD_TYPE_CONTEXT))
-#define EEKBOARD_CONTEXT_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), EEKBOARD_TYPE_CONTEXT, EekboardContextClass))
-
-typedef struct _EekboardContext EekboardContext;
-typedef struct _EekboardContextClass EekboardContextClass;
-typedef struct _EekboardContextPrivate EekboardContextPrivate;
-
-/**
- * EekboardContext:
- *
- * The #EekboardContext structure contains only private data and
- * should only be accessed using the provided API.
- */
-struct _EekboardContext {
-    /*< private >*/
-    GDBusProxy parent;
-
-    EekboardContextPrivate *priv;
-};
+G_DECLARE_DERIVABLE_TYPE (EekboardContext, eekboard_context, EEKBOARD, CONTEXT, GDBusProxy)
 
 /**
  * EekboardContextClass: