Commiting a NPE fix that was discovered while the meeting about EDAQx

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@37506 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
call Event.setCapture on dragging only.

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@37507 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e