Added the possibility to cancel goals and fixes the preempt/ execution timeouts

[RichardvdK]

Sorry for the "big diff", but this file needed some love. :)
Implement canceling of goals as that was not possible before

The biggest changes are:

• Fixed the execution and preemption timeout timers
• Made 'outcomes' a param to make it compatible with your state machine
• Updated state names to ROS2 convention (preempt -> cancel)
• added extra intermediate internal state for preemption and used state enum instead of global vars
• added function to wait for cancelation request response with timeout

[MCFurry]

LGTM! I think we can easily point this to the forked repo?

[Timple]

I don't see anything surprising, thanks for picking this up.
I'd forward this PR to the parent repository.