Add a macro executing any macro passed in argument and saving a screenshot of...

Add a macro executing any macro passed in argument and saving a screenshot of its main frame (if any). This macro will be used by the coming stressGUI application which will be part of the GUI QA suite


git-svn-id: http://root.cern.ch/svn/root/trunk@33162 27541ba8-7e3a-0410-8455-c3a389f83636