Symbol for max number of arguments in QMetaMethod::invoke()

QMetaMethod::invoke(..) takes fixed number of arguments for execution.
Adding preprocessor macros which literaly equals this number would be
useful for writing some generic code.

Task-number: QTBUG-31821
Change-Id: Ia2faf291f3f7df44a47c3cf18f5cd587d37d7d2e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>