This adds geometric sort ordering back in for semantics nodes that don't have a sort order defined.

With this change, widgets that either have no sort order, or have an equivalent sort order, will be compared geometrically. The comparison is between the two starting corners, so it is TextDirection-aware: parent nodes that are set to have LTR text will compare upper left corners of their children, and upper right when set to RTL.

Also fixed a bug in the Transform widget that didn't mark modified nodes as needing a semantics update.