use folder-name in portal URL instead of focusedFragmentId to identify focused-upon non-regular fragment

Rolls back bad-idea introduction of `focusedFragmentId` parameter.
Instead computes focused fragment id from focusedTabID
and signals from structure to theme with a new `@allowAddTab` attribute on <navigation>
logging enhancements

adds trace logging
converts some code comments to log statements
Document that getFolderNamesForLayoutNode implementation returns List of length zero or one.

Add implementation note documenting doesn't-seem-quite-to-API-spec behavior of method implementation.
revert to addressing favorite collections by folder name