[gtk-osx] Use Command as the primary modifier

 - follow the modifier pattern of other native apps
 - don't use unmodified tool accelerators because they are in
   conflict with the global menu
[gtk-osx] GDK/Quartz does not implement gdk_colormap_new()

and Dia's can do without. For now keep the old colormap code
for X11 and win32 although it should only be necessary for 8 bit
color displays IIRC.
[gtk-osx] use gtk-mac-integration for mac global menu

Top menu integration mostly working based on
http://gtk-osx.sourceforge.net/gtk-mac-integration/index.html
wdeps.py: quick darwin port