Make the performance reporting update frequency customizable

[Calinou]

The default update frequency has been changed from 1000ms to 250ms. This makes FPS counter and monitor updates smoother and more reactive. I couldn't notice any significant performance decreases by lowering the default reporting rate to 250ms, even on a nearly empty project which ran at over 4,000 FPS on my machine.

I had to redesign the FPS counting algorithm after this one so that FPS counting no longer relies on an update frequency of 1000ms.

This implements #2794.

[akien-mga]

Would have liked @reduz to review this one before merging, because Main::iteration() is very critical for performance.

[reduz]

I reverted this, as this is not the right approach to solve the problem, and all it may cause is game stalls (which are happening already with the remote scene tree).

We learned from the remote scene tree than a different way to send this information over from the debugger is needed, probably from a thread. This needs more research before any of this should be touched.