Prevent contentEditable element from losing focus

If there is a contentEditable div AND a handsontable on the same page, I cannot copy-paste data into the div because handsontable removes the focus from the div. See the following JsFiddle for an example: http://jsfiddle.net/y5969sbk/

In this commit I add a check for the contentEditable property in order to prevent further processing of the key event.