[ui-list] Improve scrollbars behaviour.

[fabiocaccamo]

This fixes part of #293.

We can assume that the ui-list is always a vertically scrollable list, but it doesn't need always to be scrolled, so these are the changes:

• Horizontal scrollbars are always hidden.
• Vertical scrollbars are visible only if necessary.

[fabiocaccamo]

The same changes should be used for #300.

[justvanrossum]

I will try this later, but I’d like to note that the source list is an example of a list that scrolls horizontally, too.

[justvanrossum]

This indeed fixes the Windows problem ffor ui-list, but not for other scrollable items, such as the ones mentioned in #293 and #300. Let's follow up in additional PRs.