class_gtkobject.m4: Add ability to decorate generated classes
class_gtkobject.m4: Add ability to decorate generated classes

Add an optional `__FUNC_DECORATION__` (eighth) parameter to _CLASS_GTKOBJECT so
that we can use it to decorate the classes, functions and methods with
__FUNC_DECORATION__ if it is needed, so that we can use that to export the
symbols without using additional tools if the compiler does not support
automatic all-symbol exporting or export by regex, or when we want to have a
more fine-grained approach to how we want to export the symbols, as in the
case of Visual Studio builds.

GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm