Allow the page number selector to be added to the toolbar

Ok, this is not really the review Jonathan did, i did rewrite the code partially because i did not really
want to have a new observer, but the code is heavily inspired in his patch.

Also the page number selector is not in the toolbar as the review had, it'd be silly to have it in both places
by default, so if you want to have it you have to go, edit the toolbar and add the "Page Number" action to it.

REVIEW: 103427
BUGS: 279128
FIXED-IN: 4.9.0