.icon-item-type is now global, with extra styling to handle row selection when
it's inside the item tree.
getImageSrc() is still used for progress windows, which is probably OK for now
because those don't stay on screen for very long. I've updated it to return SVGs
based on the current UI color scheme.
Other code uses treeitem-* images directly; getting rid of those references will
cause a lot of conflicts and be a bit more involved than this initial pass, so
I'll put that in a separate PR.
a2f259d14c