Application: Use _WRAP_CTOR() and _WRAP_CREATE() instead of _WRAP_METHOD().

* gtk/src/application.hg: This adds a protected constructor and makes the
create() method just call that, which is the normal ways to do things.
Maybe we had wrapped gtk_application_new() directly before because it maybe
once did more than call g_object_new(), but that is fine now.
Now people can derive from this class.

This fixes bug #637445 (Yannick Guesnet)