Get rid of 'global_toolbox_factory' and manage everything dock-related
with 'global_dock_factory'. The whole of 'global_toolbox_factory' was
a big special-case and getting rid of it makes it easier to extend the
session management with e.g. single-window mode dock functionality.

To get rid of 'global_toolbox_factory' we, roughly, have to
 * Replace 'global_toolbox_factory' with 'global_dock_factory'
   everywhere. We can also get rid of lots of code that did special
   things for the "toolbox" factory.
 * Make the use or interaction with the toolbox explicit in some
   places. For example, a function gimp_dock_window_has_toolbox() has
   been introduced.
 * Make GimpSessionInfoDock have an 'identifier' parameter so we can
   differentiate between the "gimp-dock" and "gimp-toolbox" dock
   types.