Only allow properties inside objects, this makes it easier to
iterate the object, which is needed for efficiently processing
control streams.
Add a choice type to mark variable properties.
SPA_TYPE_Enum -> SPA_TYPE_Id to avoid confusion with choice enum
Make it easier to allocate and initialize properties on the stack
Make more efficient methods to make objects.
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.
