Speech palette: don't save values for every slider movement

The slider trigger changed signals for every movement,
save when he user stop moving the control.

Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>
Avoid mixing signals received by settings updates - Fixes #4759

Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>
Merge branch 'speech_fix' of https://github.com/godiard/sugar into godiard-speech_fix