Enable automatic show/hide vertical scroll bar for PanelTrackViewer

[basvandenberg]

The PanelTrackViewer has the option to allow for vertical scrolling with the enableScrollY function (possible with fixed-height tracks only). This will always show a scroll bar on the right side of the track viewer, also if the track viewer fits the screen, and vertical scrolling is not needed. It would be nice if there is an option to show the vertical scrollbar only if needed, for example by providing a scrollbarAlwaysShown parameter:

panel.enableScrollY = function(scrollbarAlwaysShown = true) {
    // ...
}