Frame Limiter Configuration

Added a configuration setting to either disable (<= 0) or set the frame limit to something other than 60.
Also added an optional callback to UISlider to customize the text displayed on the tracker. I used this to make a slider in the settings for the frame limiter that goes from 30 to 200 and then to "Off".
Instead of explicitly checking for `null`, I would create a default label function that does the String-based formatting. I would also add an explicit method to reset the label function to default (if that is ever required) instead of passing `null` to the setter.

If that's to complicated, you might want to document that passing `null` resets the function to default.