SEARCH: add wait cursor when search is performed

This adds a wait cursor to the page as long as search requests are still done. This improves the feedback given to the user about the search; especially when the query has 0 results.

It might also help to detect the evergreen https://github.com/geoadmin/mf-geoadmin3/issues/1190. If there is no wait cursor, we know that the requests are not send.

The solution with the counter is sub-optimal. But considering that we want to replace the search component in the near future, it should be short-lived anyway.