iface-modem-voice: fix ordering of D-Bus signals for call addition/deletion
To have proper ordering in the D-Bus signals, the skeleton's property changes must be flushed before the Call{Add,Delet}ed signals are emitted. Without this flush, the emission of the PropertiesChanged signal is delayed until the main loop is idle. This causes problems on the client side, for example the CallAdded signal being received before the Calls property contains the call.
Closes #81 (closed)