add "expand all" and "collapse all" options

[jonorthwash]

It would be nice to be able to expand the entire tree and collapse the entire tree. I can imagine some use cases where this would make things much easier.

[codeofdusk]

"Collapse all" can be achieved by collapsing the root node, at least on WinTreeCTRL.

[jonorthwash]

Upon expanding again does everything start from a collapsed state (as "collapse all" would), or does it keep its previous state?

[codeofdusk]

If a node is collapsed and then expanded, all descendants will be collapsed.