Keyed physics

I think keyed particles are one of the most misunderstood features of particles. This is probably due to the very cryptic way in which keyed physics were presented in the ui before, so I decided to make things a bit easier.

Just committed...

Keyed physics refresh:
- Keyed targets in one list instead of "chaining", this opens up many more possibilities than before and is much less obscure.
- Better keyed timing possibilities (time & duration in frames).
- Looping over keyed targets list.

Other changes (this is for you pino :))
- New child setting "length" with threshold (great for guard & underfur with a single particle system)

Note to John E.
The new keyed particles allow for the effect you showed in your vid. Just create a particle system with newtonian physics for the deformed cube object with suitable random velocity and gravity etc. Then create a particle system with keyed physics to the non-deformed cube and add two keys to the keyed targets list. The first key is by default the emitter object, so all you need to do is set the second target object to be the deformed cube object. As the last step activate keyed timing and set the time of the second target to something around 10 frames to get nearly the exact effect you were looking for :)