Add option for piston downward speed

[Endernice61]

Sets the speed of the segment to move to the appropriate speed
Rename and redescribe original widget to indicate that it is the speed that the piston moves upward
Added new widget for downward piston speed
Gets downward piston speed in vbsp_config and passes it to PistonPlatform
Fixed frame perfect I/O break where the platform doesn't turn around:

• Sets desired_direction in _up and _dn in segment code
• If desired_direction is not set in segment code, calls verifyDirection with the original direction and records the time in verify_clock to determine if (strictly) future calls to _up and _dn happen before verifyDirection
• If desired_direction is set in _up and _dn, checks to make sure it is called in the same tick before preceding, otherwise pushes the call to the end of the I/O queue
• Added verifyDirection function which resets the direction, but first checks the parameter with the last set direction, and if different, calls _up or _dn depending on the direction it needs to go in after resetting the direction