Don't use old GTK1-style typesystem

This fixes the SCINTILLA_CLASS() and IS_SCINTILLA() macros on GTK3.
No harm on GTK2, those macros are available since 2.0.

This also makes those macros more consistent with the SCINTILLA()
macro that already uses the proper GObject calls.

Backported from Scintilla HG: 9cd7cf1d9af73d50b0423ed34a6693bbf7f57ac8

scintilla/include/ScintillaWidget.h

@@ -16,8 +16,8 @@ extern "C" {
 #endif
 
 #define SCINTILLA(obj)          G_TYPE_CHECK_INSTANCE_CAST (obj, scintilla_get_type (), ScintillaObject)
-#define SCINTILLA_CLASS(klass)  GTK_CHECK_CLASS_CAST (klass, scintilla_get_type (), ScintillaClass)
-#define IS_SCINTILLA(obj)       GTK_CHECK_TYPE (obj, scintilla_get_type ())
+#define SCINTILLA_CLASS(klass)  G_TYPE_CHECK_CLASS_CAST (klass, scintilla_get_type (), ScintillaClass)
+#define IS_SCINTILLA(obj)       G_TYPE_CHECK_INSTANCE_TYPE (obj, scintilla_get_type ())
 
 typedef struct _ScintillaObject ScintillaObject;
 typedef struct _ScintillaClass  ScintillaClass;