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-element.h

@@ -30,22 +30,7 @@
 
 G_BEGIN_DECLS
 #define EEK_TYPE_ELEMENT (eek_element_get_type())
-#define EEK_ELEMENT(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), EEK_TYPE_ELEMENT, EekElement))
-#define EEK_ELEMENT_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), EEK_TYPE_ELEMENT, EekElementClass))
-#define EEK_IS_ELEMENT(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), EEK_TYPE_ELEMENT))
-#define EEK_IS_ELEMENT_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), EEK_TYPE_ELEMENT))
-#define EEK_ELEMENT_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), EEK_TYPE_ELEMENT, EekElementClass))
-
-typedef struct _EekElementClass   EekElementClass;
-typedef struct _EekElementPrivate EekElementPrivate;
-
-struct _EekElement
-{
-    /*< private >*/
-    GObject parent;
-
-    EekElementPrivate *priv;
-};
+G_DECLARE_DERIVABLE_TYPE (EekElement, eek_element, EEK, ELEMENT, GObject)
 
 struct _EekElementClass
 {