[fix][ui] Avoid speed factor change when going previous/next frame

Going previous/next frame changes animation mode to 'Snap To Timesteps'.
This will set the speed factor text to 'All frames'.
Currently, user has to set back themselves the desired speed factor using the combo box.

This commit restores previous animation play mode after having snaped to previous/next frame,
so that user does not have to set it themselves again using the speed factor combo box.

Also, if we are in 'Real Time' play mode with for example a speed factor 'x5',
and if we click Next frame, the speed factor will blink to 'All frames' before going back to 'x5'.
As this blink is noisy, we block all animations signals to keep it unchanged during operation.