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