chore(windows): move various headers to common

* moved ConvertUTF.h -> common/windows/cpp/include
* moved crc32.h -> common/windows/cpp/include
* moved keymansentry.h -> common/windows/cpp/include
* moved registry.h -> common/windows/cpp/include
* moved unicode.h -> common/windows/cpp/include
* moved xstring.h -> common/windows/cpp/include

and updated corresponding references. Deleted a bunch of old references
to missing headers from .vcxproj files. Updated vkeys.h and vkeys.cpp
references in .vcxproj to use $(KEYMAN_ROOT) instead of relative paths.