method overwriteParams ignores required parameters
Thru the changes in tx_musicview_base those parameters, that are marked are required in the
configuration array can't be overwritten with new values set by the search interface.
This behaviour limits the search interface's possibilities, so we should add a new parameter
to the method overwriteParams that modifies the required flag:
$flexReq = $paramLocation['req'];
Inheritance is also a possibility, but that would result in duplicating code and changing only a
few lines => so we use an additional parameter.