Active/Passive scrollbars for scene2d ScrollPane

With active scrollbars (default), scrollbars can be manipulated to
scroll the pane as currently, while passive scrollbars act as mere scroll position
indicators and cannot be directly manipulated.

Added methods setActiveScrollbars(boolean) and isActiveScrollbars()

Passive scrollbars can be used when only flick scrolling is desired, and when combined with setScrollbarsOnTop(true) and a conviniently padded nine patch can be used to make the scrollbars appear on the top or left sides of the ScrollPane