The existing advice in the documentation to "simply" register the
"org.freedesktop.DBus.Properties" interface if you want to handle
properties asynchronously is pretty unreasonable.  If you want to handle
this interface you have to deal with all properties for all interfaces
on the path, and you have to do all of the checking for yourself.  You
also have to provide your own introspection data.

Introduce a new convention for dealing with properties asynchronously.

If the user provides NULL for their get_property() or set_property()
functions in the vtable and has properties registered then the
properties are sent to the method_call() handler.  We get lucky here
that this function takes an "interface_name" parameter that we can set
to "org.freedesktop.DBus.Properties".

We also do the user the favour of setting the GDBusPropertyInfo on the
GDBusMethodInvocation for their convenience (for much the same reasons
as they might want the already-available GDBusMethodInfo).

Add a testcase as well as a bunch of documentation about this new
feature.

https://bugzilla.gnome.org/show_bug.cgi?id=698375