Fix rename interval SEGV

.. when called from popupMenu.
LTM tooltip for groupby weeks is misleading

.. it lists start date for this week to start date for next week
.. now changed to say 'Week commencing <date>'.

Fixes #501.
Fix custom date range muddle when groupby weeks

.. the start date would progressively change as you selected a
date range in the sidebar. This is because the code to set the date
range to the start of a week looked at the wrong date variable when
using a custom date range.

Fixes #500.