commands: remove separate direction arg from step commands

commands: remove separate direction arg from step commands

In the internal property API for modifying a value up/down, there was
both a step size parameter and separate STEP_UP / STEP_DOWN
operations. This was mostly redundant; anything communicated with the
latter distinction can be communicated with a negative step size.
Combine the M_PROPERTY_STEP_UP and M_PROPERTY_STEP_DOWN operations
into a single M_PROPERTY_STEP operation. This is mostly an internal
change, but there is at least one user-visible difference: the
step_property and step_property_osd commands no longer have an
optional third argument to indicate direction. I think this feature
was not commonly used.