css: Handle font-size changes properly

Changing font size does not cause widgets to update their size
properly anymore, since we forgot an invalidation flag. The
problem can be observed e.g. by running gtk3-demo and calling

gsettings set org.gnome.desktop.interface font-name "Cantarell 22"
Avoid a crash in testcalendar

When changing between months, we can get into a situation like
'February 30", which GDate doesn't accept. Don't crash in that
case.
GtkCalendar: Slightly change overflow behaviour

If the first of the month was falling on a Sunday, we would not
render any days of the previous month, and instead show two weeks
of the next month at the bottom. Improve this by showing one week
of each.

https://bugzilla.gnome.org/show_bug.cgi?id=301835