WinRT: allowed 'roaming' and 'temp' folder paths to be retrieved on WinPhone 8.1

Windows Phone 8.0 either did not define, or provide access to, a 'RoamingFolder'
or 'TemporaryFolder' for apps to use.  Windows 8.0 and 8.1 do, as does
Windows Phone 8.1.  This change allows SDL-based Windows Phone 8.1 apps to
access these folders, via either the SDL_WinRTGetFSPathUNICODE() or
SDL_WinRTGetFSPathUTF8() functions.

SDL_GetPrefPath(), which on WinRT, is based on SDL_WinRTGetFSPathUTF8(), will
continue to return the app's 'local' folder, despite Windows 8.x
counterpart apps using the 'roaming' folder, in order to preserve compatibility
when 8.0-based Phone apps upgrade to 8.1-based Phone apps.

--HG--
extra : rebase_source : ffd5850a1329cb3e09d06633b18521f17fd82658
WinRT: fixed an invalid comment in SDL_GetPrefPath() (for WinRT)