SignalBase: Default to dynamic conversion preset, not custom values

The reason for this change is that when you initially select a
conversion from the channel config popup dialog, the threshold
will be set to "0.0V" or "0.0V/0.0V", respectively.
This is of course not what we want and the root cause is that
when no preset is selected, NoPreset is assumed instead of
DynamicPreset. This patch changes this.