fixes for EZGui and SynthDescLibGui

> I've made some improvements to the SynthDescLibGui, embedding the
> window for the currently selected synthdesc and fixing some bugs that
> occurred when a lib had no synthdescs yet.
>
> This series also includes a fix for EZGui for the accuracy of the
> number indicated in the numberboxes, by guessing the number of decimal
> places of the numberstep, and adjusting the max decimal places of the
> NumberBox.
> Maybe the method for guessing (of SimpleNumber) should move to another
> place.
>
> If everyone is fine with this, these patches could also go into the next
> bug fix release of 3.6.
>