Listbox: Support for infinite scrolling

When dealing with a large amount of options, GlListbox should provide a way to fetch more items when the bottom of the scrollable area is reached.

Accessibility criteria

• User's position in list is maintained when more results are appended
• Appended results update list item count e.g. "item 10 of 10" becomes "item 10 of 20" when 10 items are appended (should be default when <li> elements added)