msvc: disable compiler warnings when building all Externals (cl /W0), except for wxWidgets, where all warnings are fixed.

Note warnings may still be generated by code in Externals if it is included into warning-enabled code (this is the case for libusb).
Also, quiet an intellisense warning in wxWidgets caused by using __w64 for an inapplicable type.
msvc: resolve all warnings in Core.

Note: vs14 will support empty macro parameter as used by MMIO.
msvc: resolve all warnings in VideoCommon.
msvc: resolve all warnings in VideoBackends/Software.
msvc: resolve all warnings in VideoBackends/OGL.
msvc: resolve all warnings in DolphinWX.

Also, bring WiiSaveCrypted closer to modern dolphin coding style.
msvc: enable compiler warnings as errors.