[panel] Ellipsize long application titles, don't push clock around

First, simply set the ellipsize flag on the application menu labels.
Next, rework how we lay out the panel components so that the center
box is always centered and constrains the left and right, rather
than pushing it around.

Previously, as part of making the shell not obviously explode if
one had a lot of tray icons, we allowed them to push the clock over.
Instead, go back to just failing in this case; we need to exile legacy
tray icons, not be slightly less ugly.

https://bugzilla.gnome.org/show_bug.cgi?id=592640