ParticleEditor: delta multiplier and simpler new emitters. 

Two minor enhancements/commits to the particle editor:

- Added a _delta multiplier_ to Editor Properties; scaling down the delta time is quite useful for timing multiple emitters and so forth. I didn't resize the Properties panel, felt that it would steal valuable space from the emitter panel.
- New emitters have a much more basic configuration, it became a little arduous resetting emitter properties every time a new one was added. The initial emitter remains the same as before though.

Let me know if anything needs revising, assuming it's worthy.