Add a TreeView control for Web projects

Add a designer-visible TreeView control to Web projects so applications can present hierarchical data — file trees, category outlines, threaded lists, navigation — the way they already can on the desktop.

The control should show expandable parent and child nodes with optional per-node icons, support single or multiple selection, let users reorder nodes by dragging (when enabled) and edit node text in place (when enabled), and raise events when the selection changes, when nodes expand or collapse, and after a drag reorder. A node API builds and edits the hierarchy in code. This would bring Web projects to full API parity with the desktop TreeView for hierarchical-data workflows.