Add a method for handling for words not found in dictionary.

This is apparently called only once for every unique word, so repeated instances
of the same word are not handled separately. Upon changing orientation of the
device, however, all invalid words in the textbox are underlined again and this
method is called for each unique instance in order.

Change-Id: Ic57e4b9e5675bd7abd723644aa318d964f7f875f