Free/busy timebar alignment fix

This overlay for event attendees dialog makes sure top timebar above
free/busy aligns well with grid cells.  Luckily selection bar
displaying event's time is automatically aligned with the timebar.

Since there are differences among Lightning versions we needed to be
explicit about both padding and margins on the left and right side.
But it works like a charm on OS X (Postbox, Thunderbird 17), CentOS
(Thunderbird ESR10), Windows (Thunderbird ESR10, Thunderbird 17).