All-Plot - Add "Slope" and "Alt/Slope" curves

... add "Slope" curve based on Slope Data in Ride Files (with separate
new color)
... add "Alt/Slope" curve based on "Altitude" data series
... sections on "Alt/Slope" have 3 options -  1min/100 m OR 5min/500 m
OR 10min/1000 m
... color / slope assignments are defined in AllPlotSlopeCurve (to look
it up)
... if a plotted section is wide enough the slope % (distance) / VAM
(time) value is shown above
Merge pull request #1076 from Joern-R/pull_allplot

All-Plot - Add "Slope" and "Alt/Slope" curves