bus: split bus_map_all_properties into multiple helpers

bus: split bus_map_all_properties into multiple helpers

The bus_map_all_properties() helper calls
org.freedesktop.DBus.Properties.GetAll() on a given target and parses the
result according to a given property-table. This simplifies dealing with
DBus.Properties significantly. However, the function is blocking and thus
not really useful in many situations.

This patch extracts the core of this function and adds two new helpers
which directly take dbus-messages as arguments. This way, you can issue
asynchronous requests and parse the result via these helpers:

  bus_message_map_all_properties():
      This is the same as bus_map_all_properties() but takes the result
      message from a GetAll() request as argument. You can thus issue an
      asynchronous GetAll() request and then use this helper once you got
      the result.

  bus_message_map_properties_changed():
      This function takes a signal-message that was retrieved via a
      PropertiesChanged signal and then parses it like if you retrieved
      it via GetAll(). Furthermore, this function returns the number of
      matched properties that got invalidated by the PropertiesChanged
      signal, but didn't carry the new value. This way, the caller can
      issue a new GetAll() request and then parse the result.

The old function bus_map_all_properties() is functionally unchanged, but
now uses bus_message_map_all_properties() internally.