Wrapper function to refresh data asynchronously from a sync function.

So RefreshData can be called from property accessors.
A Windows.Devices.Geolocation API partial emulator.

Right now a stub, but this an a couple of more classes and structs
should allow us to leave most
of the geolocation parts untouched. Will be implemented using
Xamarin.Mobile.
Initial code for PlacePicker.

Already copied from the winclient-sdk version and removed
Windows-dependent code.
Geolocation parts missing, to be implemented next.