Input: wacom - add an eraser to DTH2242/DTK2241

plus send begin and end of express keys events for
Cintiq 13HD and DTH2242/DTK2241

Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>