Currently the width of the calendar column is solely determined
by the calendar, while other elements are ellipsized as necessary.

While that is the desired behavior for the events-, world clocks-
and weather sections, we don't want to cut off the date in the
header. However switching to bold text made that more likely in
non-English locales or when using large text, so explicitly take
it into account for the width negotiation.

GNOME/gnome-shell#2230