panel: merge statusBox into rightBox

Simplify the layout in rightBox by getting rid of statusBox, and just
putting everything into rightBox directly.

Simplify the handling of the user menu by adding it like it was a
status icon rather than special-casing it. Rename the "tray_icon"
variables to "status_area" to reflect this better.

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