QmlDesigner: Add PuppetAlive command

The puppet can be dead but we don't now. Now the puppet is sending a alive
signal. If the signal is not arriving anymore we restart the puppets.

Task-number: QTCREATORBUG-12434
Change-Id: I90edab6a45f95950fd764adcb125068659d5354b
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>