Ability to launch the program in deferred

[jcelerier]

Right now when the device is created it launches the program immediately (try with e.g. kate).
I'd like to launch the process with, say, an impulse message.

[thibaudk]

done with c6debc8
processes can also be killed using "$kill" as the write attribute of a parameter,
see https://github.com/thibaudk/score-addon-stdIO/blob/main/ping.qml#L28

I should address a proper autostart option in score, as described in
ossia/score#1141