Tree: support separate (user-defined) icons for open / closed state

[bill]

A user might want to have separate tree node icons for opened and closed states (think of Windows Explorer with the opened and closed folders). Want to have a CSS pattern/rule like:

.dijitTreeNodeContents .myIcon {
  background-image: url(...);
}
.dijitTreeNodeExpandedContents .myIcon {
  background-image: url(...);
}

The problem is that that expanded/contracted state can't be written on TreeNode.domNode because it will seep down to the children TreeNodes.

Alternately, make getIconClass() be called every time the node state changes (from expanded to collapsed), and pass in the new state.

[bill]

(In [11699]) Add "opened" parameter" to getIconClass() callback, to support separate (user-defined) icons for opened/closed tree nodes, typically opened and closed folders. Fixes #5003.

[bill]

(In [11700]) Changes spaces to tabs; had a weird editor setting in previous checkin. Refs #5003.

[bill]

(In [11719]) Default opened and closed folder icons for tundra, designed by nikolaionken (CLA on file). Refs #5003. There's a little horizontal vertical line at the bottom right of the opened folder; not sure where it's coming from (it's not part of the new image files)

[bill]

(In [11720]) Default leaf icon for tundra, designed by nikolaionken (CLA on file). Refs #5003. Also fixed rendering glitch (just had forgotten no-repeat on the image)

[bill]

(In [11730]) Use gif not png, to be nice to IE6. Refs #5003.

[bill]

(In [11797]) New folders icons from Nikolai (CLA on file). Refs #5003.