Instead of storing the dnd icon as a pixbuf store it
as a cairo surface and transfer the device scale
when converting it to a NSImage. In the pixbuf/stock/named
setters convert to a surface instead (no hidpi support yet)
- add gtkmodulesprivate.h and move stuff there from gtkprivate.h
- add gtkprivate.c and move stuff there from gtkmain.c
- add gtkwin32.c and move stuff there from gtkmain.c
- don't redefine GTK_DATADIR and friends in gtkprivate.h
- have _gtk_get_datadir() and friends on all platforms
- remove the horrid hacks where gtkprivate.h can't be included,
or must be included later due to redefinition of the compile-time
directories
