Retrieve properties and service introspection data upon device discovery

[markdryan]

The current method of only retrieving properties when ask for and retrieving service introspection data on the fly rather than at device construction complicates the code more than is needed. In addition, the method of synchronously retrieving the introspection data is causing lots of problems, see #97.

Let's fix this. There are 3 things to do

1. Add the gupnp_service_info_get_introspection_cancel method as described here (https://bugzilla.gnome.org/show_bug.cgi?id=704867).
2. Modify the service_task to allow us to queue calls to functions that are not gupnp actions.
3. Retrieve service introspection data and populate the properties hash table during device construction.

[jku]

I totally agree (and fixed number one already). I'm going to take a look at 2 and 3 and I'm labeling this "for-next" in the hopes it's quick to fix.

[jku]

Just to make sure there's no misunderstanding: I'm not going work on this issue, I just can't seem to assign or unassign issues anymore.

My recollection is that gupnp side was completed but parts 2 and 3 were not.