core: fix copyItem error for some platform
core: fix copyItem error for some platform

DirA.copyTo(Parent, DirB) where DirB is an existing directory has an undetermined behavior in different platform. For example, Windows copies the contents of DirA to DirB, while Mac copies the whole DirA to DirB.

danny0838/firefox-scrapbook,danny0838/firefox-scrapbook