SignalBase: Adjust Schmitt-trigger thresholds

SignalBase: Adjust Schmitt-trigger thresholds

Currently, the thresholds are determined by the minimum
and maximum values of a signal. From those, we derive the
high and low thresholds by using a 10% margin to min/max.

However, this approach doesn't work very well when the
measurement includes reset conditions or similar, causing
spikes that raise the min/max significantly.

Example:
sigrok-dumps/i2c/eeprom_24xx/microchip_24lc64/sainsmart_dds120_powerup_scl_sda_analog.sr

This patch changes the thresholds margins to 35%. However,
they are expressed differently: (max-min)/2 is used as the
center line, from which 15% of the amplitude (max-min) is
used as the margin. This way seems a little more intuitive
for me since the percentage given (15) is directly proportional
to the hysteresis.