WinRT: added initial SDL_TEXTINPUT support

Further support regarding IME and on-screen keyboards is pending, some of
which might not be 100% compatible with other platforms, given WinRT platform
restrictions.  An MSDN article at http://msdn.microsoft.com/en-us/library/windows/apps/hh465404.aspx
indicates that on-screen keyboard display requires that the user first tap on
a Windows/XAML text control.

This change adds basic SDL_TEXTINPUT support, with input coming from hardware
keyboards, at a minimum, and without the need for XAML integration (which is
still pending integration into SDL, by and large).