metomi/rose#21: more custom interface groundwork

This adds some enhancements and bug fixes necessary for work on a proper custom interface. Here's a summary:

 * <samp>lib/python/rose/config_editor/stack.py</samp>: much faster undo/redo
 * <samp>lib/python/rose/config_editor/panel.py</samp>: groupable, filterable panel class
 * <samp>lib/python/rose/config_editor/page.py</samp>: fixed custom widget importing

I'm envisaging the custom interface as a metadata option that looks like this:

widget[rose-config-edit:sub-ns] = some.SubClassOfSummaryDataPanel

@matthewrmshin, please review