Implement touch scrolling

[jtackaberry]

To better support the touch screen use-case, rtk should support touch scrolling.

This wouldn't be enabled by default, but when enabled, would:

1. Change any existing activation behaviour that occurs on mousedown to mouseup
2. Drag-scrolling within a viewport will trigger a scroll provided the widget underneath the mouse cursor isn't itself draggable.

Inertial scrolling should also be supported.

[jtackaberry]

Implemented in local branch.

[jtackaberry]

Released in 0.5.0-pre1