metomi/rose#21: custom interface groundwork

This pull request does some of the groundwork for the STASH interface issue - metomi/rose#21.

It implements a couple of things:
 * a rename section operation (in <samp>menu.py</samp>)
 * a speed up of the child copy and delete operations for rename
 * undo renames as a single operation by introducing a <code>group</code> attribute for stack items.
 * do not expand duplicate sections in the config editor page tree, by default

Note: the <samp>--new</samp> option is an unrelated omission.

@arjclark, please review.