Use a constant frame rate (25 fps) in test/cube demo

The old version was "synchronized" by calling Fl::check() in a loop
which resulted in the highest speed possible on any platform.

It turned out that this was way too fast on the Wayland platform as
`valerius` posted on IRC/Matrix on Nov 30, 2024. Citation:
"anyone else notice the cube demo on Wayland is mega ultra hyper speed?".

I took the opportunity to change the demo to use a timer with a given
frame rate (see constant 'FPS') to synchronize drawing.
Note: the 'speed' slider is independent of the drawing frequency.

Finally this commit adds statistics output when the 'Stats / Exit' button
is pressed like

  Iterations =  304, runtime = 12.167 sec, fps = 24.99, requested: 25

with the requested frame rate (default = 25) and allows the user to
continue to see more statistics.

To do (very likely not in 1.4.x):
 1. integrate running statistics in the display
 2. make the desired frame rate variable at runtime (new slider)

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