Add guard predicate to QSignalTransition

Currently the QSignalTransition needs to be subclassed and QSignalTransition::eventTest() to be overridden in order to have a conditional transition.

This could be made easier by adding guard predicate (similar as in the QML SignalTransition.guard) to the QSignalTransition. The transition will be triggered if the guard predicate returns true.

E.g by using lambda.

QSignalTransition(
   sender,
   &QObject::destroyed,
   [sender] { return (sender->objectName() == "foobar"); },
   nextState
);