The UI may want to change its event mask however to do this, it needs the
event handler's ID. Tweak the code to allow this to be stored and add
a command to query it.
Use the new command in the process server backend.
(Bitbake rev: f8cf2cb58b80ce74f756a11a9773b6b0e78d51ee)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>