panel: remove legacyBox, mix legacy icons in with regular

Legacy trayicons are mostly gone, so remove some of the special-casing
for them to simplify things.

Also, fix panel.addToStatusArea() to interpret its "position" relative
to tray_icon_order, not relative to the existing contents of
statusBox, so that the order that extension icons appear in does not
depend on the order they are loaded in.

https://bugzilla.gnome.org/show_bug.cgi?id=651299