- The icon itself was changed to a "search" icon. Though it kinda made sense to
  keep the "link" reference when I first implemented this, as a reference to how
  we used to link items (whereas now we select them together), in the grand
  scheme of things, it's not such a good idea and many won't likely understand.
  But everyone will understand a "search" button. :-)
- It's now next to the label displaying the number of selected items, in the
  item tree view header. This makes much more sense there. Also it makes the
  long list of bottom buttons slighly shorter.
- Finally I make this feature accessible to the channels and paths, by actually
  moving it out of GimpLayerTreeView and into GimpItemTreeView.