Toolbar API

Objectives:

* Make it possible to add items to toolbars without having to have all
  toolbars know about the items in advance
* Make it possible to specialize an existing tool and have it be used
  instead of the base implementation

Approach:

* Tools are named using a path-style category/id/ext system, making them
  selectable, the latter component being used to differentiate extended
  tools from their base classes, but is ignored during selection
* Toolbars have ToolGroups, which include or exclude tools by category or
  category/id, and order them by promoting and demoting selections of
  tools by category or category/id

Future:

* Add a way to place available but not yet placed tools in an "overflow"
  group
* Add a mode to ToolGroup to make the tools a multi-column drop-down style
  list with labels so tools with less obvious icons are easier to identify
  - and probably use this as the overflow group

Change-Id: I7625f861435a99ce3d7a2b1ece9731aaab1776f8
Merge "Toolbar API"