* populate most session item properties from create-item.lua to keep
  things more compact and readable
* use a standard naming scheme for the session item properties
* use session item properties instead of node properties in policy-node.lua
* improve policy-node's performance by converting the properties dictionary
  less times for each session item
* refactor some policy logic and make things slighly more readable
* change the accepted values for 'context' in wp_si_linkable_get_ports();
  use "input" and "output" to keep things clear, because the previous use
  of NULL and "reverse" were implying that a node has only one "standard"
  direction, but this is complicated for sinks w/ monitors and duplex nodes
* allow using monitors (which are Audio/Sink nodes in fact) as sources
* treat Audio/Duplex nodes as sinks, like p-m-s does
* respect the "stream.capture.sink" property of streams

Fixes #66