added getPaintOpacity() on svgCanvas, used it to simplify mouseDown callback on palette in svgEditor

git-svn-id: http://svg-edit.googlecode.com/svn/trunk@2430 eee81c28-f429-11dd-99c0-75d572ba1ddd
used .data() method shortcut to retrieve zoomValue

git-svn-id: http://svg-edit.googlecode.com/svn/trunk@2431 eee81c28-f429-11dd-99c0-75d572ba1ddd