Limited context to 80 chars in KMInputMethodBrowserClientEventHandler

I had previously made a change to do this in the base class but overloooked that it was also a factor in this subclass, so I refactored a little to have a single method  in the base class that handles this. (Note: the base class method also checks to be sure that the client responds to the attributedSubstringFromRange method before calling it. This used to be done only in the subclass, but it seems like a useful safeguard in case some other client fails to implement this.