extended plugin manager that now supports also the specification of an

ctor, to instantiate an object of the specified class, in the external file
(typically rootrc). This ctor is called via TPluginHandler::ExecPlugin().


git-svn-id: http://root.cern.ch/svn/root/trunk@4933 27541ba8-7e3a-0410-8455-c3a389f83636