[TreeView] [Gtk] [WPF] OnRowCollapsing and OnRowCollapsed implemented

[residuum]

Fixes issue #347

[residuum]

As I do not have a Mac, so I cannot try this change and therefore did not submit this as well.

After reading the documentation and Xwt.Mac/TreeViewBackend.cs, these needs to be added in the inner class TreeDelegate in that file after line 53:

        public override void ItemDidCollapse(NSNotification notification)
        {
            Backend.EventSink.OnRowCollapsed (((TreeItem)notification.UserInfo["NSObject"]).Position);
        }

        public override void ItemWillCollaps (NSNotification notification)
        {
            Backend.EventSink.OnRowCollapsing (((TreeItem)notification.UserInfo["NSObject"]).Position);
        }

[slluis]

Please don't include formatting changes.

[residuum]

I redid all the changes in new commits, so that not formatting changes are included in the pull request.

[slluis]

I added the Mac overrides and works fine. Thanks!