build: Bump required GTK+ version

https://bugzilla.gnome.org/show_bug.cgi?id=730598
ephy-window: Use the new support for RTL icons in GtkIconTheme

https://bugzilla.gnome.org/show_bug.cgi?id=730598
ephy-find-toolbar: Use the new support for RTL icons in GtkIconTheme

https://bugzilla.gnome.org/show_bug.cgi?id=730598