In order to make a GimpDock get hold of a GimpContext both in
single-window mode and in multi-window mode, don't make it look for a
GimpContext in a GimpDockWindow, put the context in GimpDockColumns
instead. GimpDockColumns exists both in s-w-m and m-w-m, contrary to
GimpDockWindow. Still use the GimpDockWindow as a backup though.