Implement WinRT plugin for positioning

[ChangeLog][QtPositioning][QGeoPositionInfoSource] WinRT backend added.

Change-Id: I00e7725a082aa485c3ab5afcaebd20344b48e07d
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Add snippet app to verify plugin implementation

Useful when creating a new backend to verify behavior.

Change-Id: I7a417aabad4d6dbbe494dcd384ca7af57bd78050
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>