cookies-dialog: add tooltip to remove cookie button

https://bugzilla.gnome.org/show_bug.cgi?id=735649
history-dialog: add tooltips to buttons

https://bugzilla.gnome.org/show_bug.cgi?id=735649
passwords-dialog: add tooltips to buttons

https://bugzilla.gnome.org/show_bug.cgi?id=735649