Data sources panel: dataset labels don't grow into free space

[pont-us]

• Dataset labels don't grow into free space

To reproduce: drag to reduce width of left-hand panel, and horizontal scroll-bar will appear. Scroll right with the scroll bar to confirm that dataset label did not grow into available space. One fix/workaround might be to change to "overflow: 'auto'" in ScrollableContent's style to "overflow-inline: hidden", which stops the horizontal scrollbar appearing (but best to do this by creating a new VerticallyScrollableContent component to avoid the risk of breaking the other 10 panels that use ScrollableContent).

Transferred from Issue #12, since this particular issue is not a blocker.