dateMenu: Freeze popup size while browsing

dateMenu: Freeze popup size while browsing

As the popup's height depends on its content, which itself varies
depending on the selected day, browsing the calendar can result
in distracting size changes. To avoid this, the design calls for
the height to be frozen to the previous one in that case.
As the popup will always open with the current day selected, we
don't have to be very sophisticated and can just lock the popup
to the height corresponding to that day.

https://bugzilla.gnome.org/show_bug.cgi?id=744817