dateMenu: Enforce calendar column width

dateMenu: Enforce calendar column width

We want the width of the calendar column to be determined by the
calendar, other elements should adjust their allocation accordingly.
However neither ellipsization nor wrapping will kick in unless the
parent's width is restricted, so use a small custom layout manager
that enforces the desired behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=754031