I previously used a basic 2N3906 and then changed its footprint to
SOT-23. This led to the wrong pin assignment.
This commit fixes it by using the correct symbol and footprint for it.
Note that the PCB layout still needs fixing.
Moved the Q4 photodiode footprint dot (representing the collector) so
it's more clearly visible. Hopefully it will help addressing the
recurring confusion about Q4's orientation (#3, #42).
