panel: Move statusmenu initialization in the constructor

panel: Move statusmenu initialization in the constructor

While related to the status area, the user status button is clearly
not a status icon, and it does not make too much sense in
startStatusArea(), which is about filling the status area with
icons. Also, the status icon container is added to the panel in
the constructor, in fact, the user status button is the only "toplevel"
panel element which is initialized elsewhere. Not a crucial change,
but makes for a nice read anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=642697