add widget only when hook is called

git-svn-id: http://dev.piwik.org/svn/trunk@7788 59fd770c-687e-43c8-a1e3-f5a4ff64c105
next try

git-svn-id: http://dev.piwik.org/svn/trunk@7789 59fd770c-687e-43c8-a1e3-f5a4ff64c105