RFE: (swing) Add ability to control track increment with scrollable interfaces

Swing's Scollable interface currently allows one to control the scroll
increments for unit and block. However, a customer has pointed out that
it would be helpful if "track" increments was programmable as well.

To paint a better picture:

Unit ->Clicking on one of the end arrows
Block->Clicking somewhere between the arrows and "knob"
Track->Clicking and draging the "knob"