Add a new read-only GimpToolItem::shown property, and a
corresponding GimpToolItem::shown-changed signal, which determines
if a tool item is visible throughtout the hierarchy, i.e., if it
and all its ancestors are visible. This replaces
gimp_tool_item_is_visible().
Use the new property and signal in GimpToolPalette and
GimpToolEditor, to simplify the code, and in preparation for a flat
toolbox view.
(cherry picked from commit ee9661237e)
Add GimpToolItem as a common base class for toolbox items.
Derive GimpToolInfo from GimpToolItem, representing an individual
tool. The next commits add support for tool groups, represented by
an alternative subclass of GimpToolItem.
Most of the tool-info properties remain in GimpToolInfo, however,
GimpToolItem takes care of tool-item visibility.
