Modify multicheck to accept parent parameter.

This makes it possible for multicheck to become a tree-like structure.

Example:
arg {number=4}{call=--devices}{display=Devices}{tooltip=Device selector}{type=multicheck}
value {arg=4}{value=dev1}{display=Parent Device}{enabled=t}{default=t}
value {arg=4}{value=dev2}{display=Child Device}{parent=dev1}{enabled=t}
value {arg=4}{value=dev3}{display=Another Parent Device}{enabled=t}
value {arg=4}{value=dev4}{display=Non-clickable Child Device}{parent=dev3}
value {arg=4}{value=dev5}{display=Non-clickable Child of Child}{parent=dev4}

Change-Id: I59dd7208ca0ec90cccfc49ae049559cdc6c69a4b
Reviewed-on: https://code.wireshark.org/review/4192
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Roland Knall <rknall@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>