This change adds a new class to the quicksearch's <form>
element while the quicksearch is running. This is to
make it easier to style the form, e.g. with a throbber gif
in the search box, to indicate that something is happening.
For slow connections, it is sometimes not apparent that
waiting for a few seconds will present the user with a list
of matching pagenames.

The new 'searching' class is added just before the POST
request is sent, and cleared both on successful completion
and when the search box is cleared.