Add a "Menu mode" option to the toolbox preferences, which controls
the menu behavior for tool-group buttons, and can be one of "Show
on click" (current behavior), "Show on hover" (show the menu when
hovering over the button), and "Show on hover in single column"
(behaves like "Show on hover" when the toolbox has a single column,
and "Show on click" otherwise) -- the latter is the default.
Note that "Show on hover" requires the ability to remove the menu
grab, which doesn't seem to work in GTK3, so this change is
restricted to 2.10 for now.
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.
