Remove identic implementations of wxDir::Exists from platform-specific files and...

Remove identic implementations of wxDir::Exists from platform-specific files and put it in dircmn.cpp (they all used wxDirExists).
Add wxDir::Make() and wxDir::Remove() for coherency with wxDir::Exists() and document them as simple aliases to wxFileName functions, just a bit more readable.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64632 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775