Re-organize flags for FLICKY_CENTER

* Flickies can be in-place with just one flag (MF_GRENADEBOUNCE)
* Flickies can now hop whether orbiting, aimless, or in-place
* Removed ability to customize movedir for orbit; will always be random
* Removed ability to customize gravity sink for aimless flying flickies
* Misc: Use P_SetTarget to set actor->tracer for flicky center