Add a new Gimp::tool_item_list list, in addition to
Gimp::tool_info_list.  The latter may contain arbitrary tool items,
including tool groups, and is intended for use in the UI (namely,
the toolbox and the preferences tool editor).

In gimp-tools, use Gimp::tool_item_list for representing the UI
tool order (while still using Gimp::tool_info_list as a flat list
of all GimpToolInfo objects), and add support for saving and
loading tool groups to/from toolrc.

Introduce file-version tracking in toolrc, and drop its contents on
version mismatch, or when new tools are introduced.  This is
slightly disruptive, but merging new changes with existing toolrc
files is non-trivial, and it doesn't happen very often.

Add support for a sysconf toolrc file, which is used if there's no
user toolrc file (i.e., on first use).  If neither file is found,
the hard-coded flat tool order is used.  This commit doesn't
provide a default toolrc file, but the next commits will.

Make the gimp-tools serialization and deserialization functions
public, for use in GimpToolEditor in the next commits.