Add FPS Display

[gselzer]

Description

Especially for a live data feed (think Live view from Micromanager), it might be nice to display the FPS somewhere on the window.

[tlambert03]

i don't really want to burden the public API with this any time soon. and there's enough other features that need to be implemented, that i'm gonna close this as not planned (doesn't mean i wouldn't accept a PR on it... just don't need an open issue dedicated to it)