In editor, always draw an outline for the root UIBlock if any of its children are selected

[JacekPrzemieniecki]

UGUI displays an outline for the canvas even if it's not directly selected. This helps with element placement if there is no full-screen background image in the UI, because you get a better sense of where you are on the screen.

A snippet I'm using for now, illustrating what I'd like to see added:

// in UIBlockGizmos.DrawGizmos
var root = uiBlock.Root;
if (selected && root != null)
{
    OnDrawGizmosInSelection(root, selectedColor);
}