[voicecall] Reduce the DBus overhead of call hander interface.

The declarative plugin provides properties of a call by via a
synchronous call every time the property is accessed. In QML this can
happen _very_ often.

Cache the properties on the client side, and provide property values in
change signals to eliminate the blocking calls.
Merge pull request #50 from martinjones/async_handler

[voicecall] Reduce the DBus overhead of call hander interface.