Decoupled crosshair and tooltip.

Crosshair logic now resides in Axis instead of Tooltip.
Possible to define a crosshair on the axis (preferred way).
Possible to make a 'floating' crosshair that does not snap to data
points.
//docs:
Added crosshair-label on the axis.

Stockcharts now have the option to display a label on the axis that
follows the crosshair.
Docs://
Merge branch 'crosshairs'