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).
WinRT: removed a completed TODO item from an inline comment
WinRT: added SDL_HINT_WINRT_PREF_PATH_ROOT

SDL_HINT_WINRT_PREF_PATH_ROOT allows WinRT apps to alter the path that
SDL_GetPrefPath() returns.  Setting it to "local" uses the app's
OS-defined Local folder, setting it to "roaming" uses the app's OS-defined
Roaming folder.

Roaming folder support is not available in Windows Phone 8.0.  Attempts to
make SDL_GetPrefPath() return a Roaming folder on this OS will be ignored.

Various bits of documentation on this were added to SDL_hints.h, and to
README-winrt.md
move AddDisplayt to registry handler code.