panel: center date entry with workarea

Align and center the date entry with the workspace's workarea.
This way, maximized applications have their window aligned with the top date
entry.
This doesn't change anything for desktops with no docks or when left/right
workareas are aligned with the monitor.

The offset is leftOffset - rightOffset:
(workArea.x - monitor.x) - (monitor.width - ((workArea.x - monitor.x) +
                                              workArea.width))

https://bugzilla.gnome.org/show_bug.cgi?id=792354