Rename "on axis line" to "leader line"

[pixelzoom]

Rename "on axis line" to "leader line" throughout, since this seems to be what the design team calls them. Leader lines connect a vector to the projection of its component vectors onto the x and y axes. This will make it easier to find things in the code, like when someone asks to see thicker leader lines.

E.g.:

• onAxisLinesOptions
• ON_AXIS_LINES_NON_ACTIVE_STROKE
• ON_AXIS_LINES_ACTIVE_STROKE

Would also be good to explicitly set leaderLineWidth.

[pixelzoom]

Done, closing.