Feature request: Add Heart Rate Lifeline to gameplay and density graph

[pnn64]

Add Heart Rate Lifeline to gameplay and density graph.

Keep track of a separate life line during gameplay. We should figure out a way to maybe overlay the scatterplot with heart rate zones if preferred. Maybe opposite of Barely, there should be a point in the scatterplot where we mark the HRmax with a number. Heart Rate should probably scale with the density graph so the highest point in the graph was the highest you achieved during a song

[DanPeriod]

A thinner orange line on the density graph to represent current HR is probably the easiest way during gameplay. Could color to HR zone, but then white would conflict with the lifebar line.

You theoretically shouldn't need to rescale the graph if the player's "Max Heart Rate" is set correctly, or just have the graph peak at or above the set value (don't recalc any zone ranges if you do rescale, since they wouldn't match the gameplay then.) Marking the point of the highest HR is a good idea though, and maybe some kind of indication or prompt that the player should update their Max HR if the song peak was higher than what is entered.