Switch from USING_GLES2 to MOBILE_DEVICE.

Still using USING_GLES2 for, well, GLES2.  But for things that are really
about mobile, we need a new define.  Devices are coming that don't use
GLES2.
Clean up defines in MSVC project files.

This way they're all basically the same.  A lot of cases where
debug/release/etc. where inconsistent...

Also, define USING_WIN_UI.
Use USING_WIN_UI instead of _WIN32 for differences.

Just makes the code clearer and probably helps other UIs work on Windows
more easily.
Allow PSPModel to be set/saved on mobile devices.

Fixes #5289, still defaulting off on mobile.