toolbar: Move the find button to the left

So that is grouped with the navigation actions, instead of the view
actions.

https://bugzilla.gnome.org/show_bug.cgi?id=723789
findbar: Add close button to the find bar

It makes more obvious how to close the find bar, specially when it has
been launched by a keyboard shortcut instead of using the toggle button
in the toolbar.

https://bugzilla.gnome.org/show_bug.cgi?id=723789