sink-input, source-output: rework property setting

sink-input, source-output: rework property setting

pa_sink_input_update_proplist() is inconvenient in many cases, because
it requires allocating a new proplist, even if the goal is to just set
one property. pa_sink_input_update_properties also can't properly log
property changes, because it has to assume that all values are
arbitrary binary data.

This patch adds pa_sink_input_set_property() for setting a string
value for a single property, and pa_sink_input_set_property_arbitrary()
for setting a binary value for a single property.
pa_sink_input_update_properties() is reimplemented as a wrapper around
pa_sink_input_set_property_arbitrary() to centralize logging and
sending change notifications.

(The above mentions only sink input functions for brevity, but the
same changes are implemented for source outputs too.)