Add a new GimpToolButton class, used for tool-buttons in the
toolbox, instead of implementing them directly in GimpToolPalette.
Each GimpToolButton is associated with a GimpToolItem, which can be
either an individual tool or a group.
When a tool button is associated with a group, it displays the
group's active tool, with an arrow at the corner. Clicking the
button selects the active tool, while clicking-and-holding, or
right-clicking, shows a menu of all the tools in the group.
Alternatively, the active tool can be changed using the scroll
wheel.
