Add selection mode to contextMenu.

git-svn-id: http://primefaces.googlecode.com/svn/primefaces/trunk@11870 5a70e874-a5cb-11dd-8af2-493fb0bb9f7c
Fixes issue 7469

git-svn-id: http://primefaces.googlecode.com/svn/primefaces/trunk@11872 5a70e874-a5cb-11dd-8af2-493fb0bb9f7c