Simplify persist

[XeroOl]

For func, and also for perframe and func_ease. perframe and func_ease are slightly different.

New Behavior
persist=
When true (default), the function will always be called.
When false, the function will only be called if at least one frame has happened before beat.
When its a number, the function will only be called if at least one frame has happened before beat + persist.
(The function is always guaranteed to run in gameplay, but will be skipped in the editor if the part being played is more than persist away from the func)

Current Behavior
When true (default), the function will always be called.
When false, the function will only be called when a frame happens between beat and beat + 0.5.
When its a number, the function will only be called when a frame happens between beat and beat + persist.
(The function is guaranteed to either run between beat and beat + persist, or not at all)

[XeroOl]

I don't believe there is high demand for this, so it is probably not worth the effort. If anyone was looking forward to this feature in v6, please let me know because right now I think this will get skipped