Finally remove Old Ride list QTreeWidget !

.. its been there since the beginning at the heart of the code
   as a registry of the rides (RideItem) and controlling the
   selection of rides.

.. in v3.0 we stopped showing it, but it was still created and
   then hidden immediately. But removing the 'spine' of the code
   was seen as a step too far.

.. this is the first part of moving from SQL to a NoSQL cache
   for ride metrics, metadata and measures -- RideItem is now
   no longer inheriting from QTreeWidgetItem with all the issues
   that brings.

.. since its right at the heart there are likely to be unforeseen
   bugs as we go, especially since it affects the ride navigator.

.. add/delete has been tested a fair amount and should be reliable.
Save Command flushed Metadata first

.. to fix long standing bug with saving whilst editing
   a field. But still leaves a problem if exiting whilst
   editing a field (which is a bit weird tbh).
Add RideMetric::toString(bool useMetricUnits)

.. so we can now call that instead of doing the conversion
   and formatting all over the code !

NOTE: it still needs to be /called/ in the code, that change
      will need to be applied everywhere a metric is displayed
      to the user.
Merge pull request #1185 from amtriathlon/master

Added xPace metric
Fixed copy and paste error