[D-Bus] Add support for GetAll() in PropertyExporter

Some code was refactored so we could reuse between Get() and
GetAll(). The function AppendPropertiesToWriter() is being exposed
publicly so we can use it to implement GetManagedObjects() in the
future.
Merge pull request #1184 from cmarcelo/dbus-get-all

[D-Bus] Add support for GetAll() in PropertyExporter