Improve API to transform all children nodes

[pleonex]

Transform children nodes

var overlays9 = context.GetSystemNode("overlays9");
foreach (var overlay in overlays9.Children) {
    _ = overlay.TransformWith(compressionConverter);
}

Proposal:

• TransformChildrenWith(bool recursive)
• Extension method for LINQ (see below)

Transform children nodes with filter

var overlays9 = context.GetSystemNode("overlays9");
foreach (var overlay in overlays9.Children.Where(n => n.Name.StartsWith("ov9"))) {
    _ = overlay.TransformWith(compressionConverter);
}

Proposals:

• Extension method for LINQ: overlays9.Children.Where(...).TransformWith<T>()
• Do the same for NodeContainerFormat so it works without nodes.
  • Maybe not needed if they do containerFormat.Root.Children