Color coded gutter markers

[geoffxy]

We currently show gutter markers for lines of code where we have performance information. However we use the same color, even if the line of code contributes very little to the overall performance footprint. We should consider using different colors or shades to indicate the "weight" of the line of code.

[geoffxy]

We need to scope the relative color coding by function. We can do this by storing a set of each file path and line number combination per node. Note that this information is already available somewhat by considering the contexts of each breakdown node's children.