Added limited message-box support for Windows Phone 8.1

The Windows Phone 8.1 'MessageDialog' API only seems to support two buttons,
despite the documentation for such mentioning support for three.  Trying to use
three or more buttons leads to an exception being thrown.  As such, any attempt
to use more than two buttons via SDL_ShowMessageBox (on Windows Phone 8.1) will
lead to no message box getting shown, and the call returning an error.

The Win32 MessageBox and dialog APIs are not available in WinRT apps, to note.

More extensive message dialog support might be available at some point, if and
when XAML support is more fully fleshed-out.  I'm not certain of this, though.

--HG--
extra : rebase_source : 42962c17f0bc3093e67fe9e176fb7c7c4bae722c
WinRT: fixed the max-button count for WinPhone 8.1 message boxes

Doh.

The max button count on WinPhone 8.1 was set to three, not two.  Any more than
two leads to a crash.