types: Clean up gtkwidget.h includes

In particular gtksettings.h and gtkstylecontext.h needed to be included
in lots of places now.

Also, I order the includes alphabetically in a bunch of headers.

gtk/gtksettings.h

@@ -23,6 +23,7 @@
 #define __GTK_SETTINGS_H__
 
 #include <gdk/gdk.h>
+#include <gtk/gtktypes.h>
 
 G_BEGIN_DECLS
 
@@ -37,17 +38,11 @@ G_BEGIN_DECLS
 
 
 /* --- typedefs --- */
-typedef struct _GtkSettings GtkSettings;
 typedef struct _GtkSettingsPrivate GtkSettingsPrivate;
 typedef struct _GtkSettingsClass GtkSettingsClass;
 typedef struct _GtkSettingsValue GtkSettingsValue;
 
 
-typedef gboolean (*GtkRcPropertyParser) (const GParamSpec *pspec,
-                                         const GString    *rc_string,
-                                         GValue           *property_value);
-
-
 /* --- structures --- */
 struct _GtkSettings
 {