Checkpoint ComparePane now toggles off/on

.. added a button to switch compare on and off.
UI Nits: Fusion Style on QT5 only

.. improves things somewhat on Windows.
MergeActivityWizard: don't crash if no data to merge
Compare Mode Connected Up

.. Connected the compare mode on/off switch in the
   compare pane to Context to notify charts

.. the scope bar and chart painting now highlight in
   red when compare more is active and supported by
   a particular chart.

.. each chart must implement isCompare() to return
   true or false if it is supporting compare mode.

We /still/ do not have a mechanism for clearing or
reordering items in the compare pane, nor a mechanism
for choosing 'delta' compare (i.e. baseline all compares
to first item in the list).
RideSummary Compare Mode

.. you can now compare intervals or date ranges via
   the ride summary window.
Compare Seasons fixups

.. update the title

.. don't keep adding ride count to the total
   metrics to calculate

Need to work out what to do with;
* Temperature
* Athlete Bests
RideSummary Honours CheckBoxes

.. if intervals or date ranges are not checked in the
   compare pane then we shouldn't summarise them

.. also fixed up compare pane so first entry ie. what we
   are comparing to is always checked.
Handle Compare Pane Sort

.. if the user sorts the compare pane contents by clicking
   the header the order they are stored is changed too.

NOTE: The table sort is broken since it is alpha not numeric!
      This needs to be fixed too.
Fix compare pane sorting

.. sorts according to the expected format for each column
   but if in doubt assumes its numeric
merge upstream into master