Add FPS throttling (currently hardcoded to 10 FPS)

Previously frames were being painted on every single call to
tuikit.Paint(). This resulted in considerable waste of resources when
many frequently changing sources were databound to UI elements. Each
datasource change resulted in a repaint. See
`_examples/databinding/databinding.go` for an example.

Now tuikit calculates the number of frames per second (FPS) and drops
frames above 10 FPS. Calling tuikit.Paint() more frequently than 10 FPS
causes only negligible overhead.

In my tests I found 10 FPS to be good enough for terminal applications.
But I might make it a configurable flag instead of the current hardcoded
preset.

As a side product of the above change the FPS calculation (`fps.go`) has
also been simplified.
Set random sleep interval outside of loop