dateMenu: Do not ellipsize date header

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.

Fixes #2230 (closed)