Move code in test-session-management.c to a common place so we can
have several variants of session management tests. We can't easily run
all session management tests in a single process, because some parts
of Gimp (the GObject) initialization eeks if done twice in the same
process.
Rename test-session-management.c to test-session-2-6-compatibility.c
and create a new test for GIMP 2.8 multi-window mode session files,
namely test-session-2-8-compatibility-multi-window.c. Copy and rename
sessionrc files accordingly.
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.
