Tree: implement icons

[bill]

The tree control should be able to put icons to the left of each label. Icons should be specified in CSS as background images (they won't show up in high-contrast mode but that's OK because the text is still there).

User defines a class for each icon (just like Button and Menu icons) and then defines a function that generates the icon class based on the dojo.data.item and the node expanded/closed state.

Maybe a default code like

iconAttr: "icon",

getIconClass: function(store, item, /*Boolean*/ opened){
   return store.getValue(item, this.iconAttr);
}

(For folder nodes there can be two icons, one for when it's expanded and one for when it's contracted, so we call the function every time we change state.)

[bill]

(In [10265]) Implement icons for tree (fixes #3314). Still need to support change of icon for expanded/contracted state (ie, opened/closed folder)