Implement native message box for WinRT

Added a basic theme class, which creates accessors for the
QPlatformMessageDialogHelper. This handles creation of the
native dialog, but also spawning an event loop around it.

There are some limitations shared with the Android implementation.
First you cannot create custom labelled buttons and also selecting
the default button is not possible. This will be added at a later
point.

Note that Windows Phone is not supported.

Change-Id: I2dec83dcded7919835a6dcdf8dc2a56282024a7e
Reviewed-by: Andrew Knight <andrew.knight@digia.com>