Add GimpDockContainer::get_dock_side() and write that info in the
sessionrc file. In single-window mode, a dock can be either on the
left or right side of the image window. In a dock window however, the
side concept doesn't apply ('side' has nothing to do with what column
a dock is in in a GimpDockColumns)
Make GimpDockContainer::get_docks() return a new list and not just a
pointer to a list so that GimpImageWindow can implement it (who needs
to merge its two GimpDockColumns lists).
We now have to classes that contain docks, GimpDockWindow and
GimpImageWindow (in single-window mode). Introduce a GimpDockContainer
interface so we can cope with these in an abstract way.
