84179434 speed slider click to jump

New pull request against development branch:
Feature request [#84179434](https://www.pivotaltracker.com/story/show/84179434): Make clicking on blockly Speed Slider bar move the selector to that location.

Implemented so that when you click on the track (not the knob or elsewhere on the SVG) the knob will jump to the mouse position, then begin a drag action as usual.