Implement CDU keys as switches instead of using buttons

Right now, for CDU keypresses the plugin only receives the "down" command and ignores the "up" command. This does not allow to detect longer presses, as for example for the CLR key. Also it creates a lot of "unknown command" entries in the log file.

Implementing CDU keypresses as switches with pushbutton flags would send "Start" and "End" commands and allow detection of button holds.