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)

eek/eek-key.h

@@ -31,31 +31,7 @@
 G_BEGIN_DECLS
 
 #define EEK_TYPE_KEY (eek_key_get_type())
-#define EEK_KEY(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), EEK_TYPE_KEY, EekKey))
-#define EEK_KEY_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), EEK_TYPE_KEY, EekKeyClass))
-#define EEK_IS_KEY(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), EEK_TYPE_KEY))
-#define EEK_IS_KEY_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), EEK_TYPE_KEY))
-#define EEK_KEY_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), EEK_TYPE_KEY, EekKeyClass))
-
-typedef struct _EekKeyClass EekKeyClass;
-typedef struct _EekKeyPrivate EekKeyPrivate;
-
-/**
- * EekKey:
- *
- * Contains information about the state of a key.
- * TODO: rewrite as a plain struct
- *
- * The #EekKey structure contains only private data and should only be
- * accessed using the provided API.
- */
-struct _EekKey
-{
-    /*< private >*/
-    EekElement parent;
-
-    EekKeyPrivate *priv;
-};
+G_DECLARE_DERIVABLE_TYPE(EekKey, eek_key, EEK, KEY, EekElement)
 
 /**
  * EekKeyClass: