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-symbol.h

@@ -59,27 +59,7 @@ typedef enum {
 } EekSymbolCategory;
 
 #define EEK_TYPE_SYMBOL (eek_symbol_get_type())
-#define EEK_SYMBOL(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), EEK_TYPE_SYMBOL, EekSymbol))
-#define EEK_SYMBOL_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), EEK_TYPE_SYMBOL, EekSymbolClass))
-#define EEK_IS_SYMBOL(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), EEK_TYPE_SYMBOL))
-#define EEK_IS_SYMBOL_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), EEK_TYPE_SYMBOL))
-#define EEK_SYMBOL_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), EEK_TYPE_SYMBOL, EekSymbolClass))
-
-typedef struct _EekSymbolClass EekSymbolClass;
-typedef struct _EekSymbolPrivate EekSymbolPrivate;
-
-/**
- * EekSymbol:
- *
- * The #EekSymbol structure contains only private data and should only
- * be accessed using the provided API.
- */
-struct _EekSymbol {
-    /*< private >*/
-    GObject parent;
-
-    EekSymbolPrivate *priv;
-};
+G_DECLARE_DERIVABLE_TYPE(EekSymbol, eek_symbol, EEK, SYMBOL, GObject)
 
 /**
  * EekSymbolClass: