Cleaned up and rearranged WinRT project file structure.

All WinRT projects have been merged into a single directory, "VisualC-WinRT",
with platform-specific variants in subdirectories off of it.  This structure
has been applied to a few major SDL satellite libraries as well (SDL_image,
SDL_mixer, and SDL_ttf).

Currently, only Windows 8.0/RT and Windows Phone 8.0 targets are supported.
Windows 8.1/RT/Phone targets are planned.

Projects that use SDL_image/WinRT, and link to it via Visual Studio's
project-to-project reference system, will need to be updated, to reflect the
changes in the project structure.  This can be done by:
1. removing the MSVC project(s) for SDL/WinRT
2. re-added the MSVC project(s) for SDL/WinRT
3. right-clicking on the app, or projects that use those libraries, choosing
References, removing the references to any of these projects (they'll likely
be highlighted with an exclamation mark), then re-adding them

To note, the satellite libraries that reference SDL/WinRT have been updated
already.  The changes for those libraries will be pushed to hg.libsdl.org
shortly.

TODO:
- add support for Windows 8.1 and Windows Phone 8.1 targets, using
Microsoft's new "Universal" app support, if possible.  These will be added to
a new subdirectory, or subdirectories (if more than one sets of projects are
needed, hopefully not), of "VisualC-WinRT".
- investigate NuGet support, which could allow Visual C++ to download
new copies of SDL/WinRT, its satellite libraries, and their dependencies,
from remote servers.