Add the functionality of key sequences 'cw', 'ce', 'cb', 'dw', 'de' and 'db'.
Implemented move_to_beginning_of_next(), move_to_end_of_next(), move_to_beginning_of_previous() and move_to_end_of_previous() functions for more correct word jumping than the move_to_xxx_span() methods that were previously used.