Reorganize the io parameter ids and objects. Make separate enumerations
for buffer, control, input and output properties.
Add a volume output property to export-source. This is still unused
but will eventually be routed to a PropsIn io area where it can
control the volume of a mixer, for example.
Pass a spa_pod as the control io area. The pod is not supposed to
change in structure, only the values do so we can achieve the same
kind of performance as passing a single value but still allow for
more complex io area layouts to be passed later.
Add control properties to audiotestsrc for frequency and volume
and add a test program.
Add size parameter to set_io for additional checks and to make it
easy to know how much control data we can write.
