Fix dropdown scrolling to top when the virtual scroll plugin fetch next page

[MatTheCat]

Fixes #556 (closed for inactivity but still present).

I stumbled upon two distinct issues when investigating this one:

• the lastQuery property was used for checking the UI staleness, as such it was often reset independently of the search query. When the virtual scroll plugin triggers refreshOptions the UI is stale so the first option is activated and scrolled to, while it shouldn’t happen since the query didn’t change. This is fixed by splitting the staleness check in a isDropdownContentStale property, which is updated independently of lastQuery.
• the “loading_more” element is made so that you can select it when navigating using the keyboard. Problem is, it will be removed by refreshOptions which then won’t find it in the dropdown and activate — and scroll to — the first option.