WinRT: made Win[Phone] 8.1 projects be structured like their 8.0 counterparts

This change removes the "Shared" component from the Windows 8.1 and Windows
Phone 8.1 project files, then renames the projects to use a file structure
that's the same as  used for the Windows 8.0 and Windows Phone 8.0 projects.

This change now places WinRT projects in the following directories:

VisualC-WinRT\WinRT81_VS2013\ -- Windows 8.1 project files
VisualC-WinRT\WinPhone81_VS2013\ -- Windows Phone 8.1 project files (NEW, as of this change)
VisualC-WinRT\WinRT80_VS2012\ -- Windows 8.0 project files
VisualC-WinRT\WinPhone80_VS2012\ -- Windows Phone 8.0 project files

Windows 8.0 and Windows Phone 8.0 projects, as well as apps or libs that
reference these, should be unaffected by this change.

Windows 8.1 and Windows Phone 8.1 based apps or libs that reference SDL's
projects directly will need to have their old references removed, then
replaced with new ones that point to the updated structure.