Improve QDoc default parameter handling for qmlmethod

Currently in order to use parameter marker (\a) with a default parameter in a \qmlmethod the only way to mark this parameter seems to be with both the variable name and the defaulted value. Example:

\qmlmethod void Sample::function(int i=0) 
\brief this is brief with an \a i=0.

Optimally it should be possible to mark the parameter without the defaulted value. Example:

\qmlmethod void Sample::function(int i=0) 
\brief this is brief with an \a i.