profile: use dynamic distances between labels on chart

Use variable intervals for printing temperature and heart
beat labels. Obviously, so that the labels don't become
sparse on zooming, but also to make them not too crowded
on mobile / small screens.

This doesn't work for depth labels, because these labels
use data provided from the profile.c core that doesn't
know about the size of the chart.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

profile-widget/divecartesianaxis.h

@@ -45,6 +45,9 @@ public:
 	double width() const; // only for vertical axes
 	double height() const; // only for horizontal axes
 
+	// The minimum space between two labels on the plot in seconds
+	int getMinLabelDistance(const DiveCartesianAxis &timeAxis) const;
+
 private:
 	Position position;
 	bool inverted; // Top-to-bottom or right-to-left axis.