Improve behavior with arbitrary display refresh rates

[kpreid]

The simulation steps are currently always 60 Hz, and the display refresh logic somewhat (depending on which windowing interface is being used) assumes that the display will also be interested in receiving frames at 60 Hz. One user has reported to me that it behaved oddly on their (I think) 120 Hz display.

The frame timing logic should be redesigned to explicitly accommodate whatever the refresh rate actually is.

A place to start on implementing this without needing new test hardware would be to change the simulation rate away from 60 Hz, which is something I want to support anyway.

[kpreid]

Update: I have decided, and partially implemented, that Universes shall have explicitly selected simulation rates. Rendering therefore must mediate between that and the display refresh rate.