QHeaderView in Athlete.h for Jaime.
PfPvPlot honour ride plot background

Similar to fixups to CP plot for user defined plot backgrounds
this update updates the PfPv plot to also re-use the user prefs
when setting axis / canvas and marker colors.
Histogram Chart honour Plot Background Prefs

.. more of the same as done for AllPlot, CP, PfPv.
Fix 3d grid when users specify background colors