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.
1093462ae9