Add support in QML State for running a StateChangeScript when exiting a State

It is now possible to define a StateChangeScript that is run when a state is entered. But it is not possible to run a script when leaving a State.

A State could have properties "onEntry:" and "onExit:" that would contain the name of the script to run upon entry and exit.

I can work around this by adding script actions to Transitions, but then I possibly need to add the action to several transitions (if a state has more than one possible next states). And a transition is not the right place to do state related clean up/housekeeping actions, anyway.

As an example, UML state machines support entry and exit actions: http://en.wikipedia.org/wiki/UML_state_machine#Entry_and_exit_actions