Tabbed Athlete Fixups

* showhideToolbar isn't available on a Mac
* used showhideToolbar for Tabbar (typo)
* setDocumentMode for tabbar on Mac (slightly better)
Show/Hide tabbar as needed

.. only needed when we have >1 tabs so lets hide and
   show it automatically.

.. also added a setting for remembering setting but it
   doesn't do anything as we auto hide/show.
Remove tabbar from context save/restore

.. when switching tabs -- as it makes no sense
   at all to hide/show tabbar as you switch tabs!
AllPlot: Add secondary Scope to plot speed and headwind in the same together
GcChartWindow: Add "export as PNG" in menu
fix #738
Export as PNG : hide menu button
merge upstream into master