Add winstring.h, hstring.idl and corresponding runtimeobject imports

git-svn-id: https://mingw-w64.svn.sourceforge.net/svnroot/mingw-w64/trunk@5879 4407c894-4637-0410-b4f5-ada5f102cad1
winbase.h: Added some missing declarations.

git-svn-id: https://mingw-w64.svn.sourceforge.net/svnroot/mingw-w64/trunk@5880 4407c894-4637-0410-b4f5-ada5f102cad1