calendar: Do not overload :active to mark selected day

calendar: Do not overload :active to mark selected day

We currently use the :active pseudo class to mark the selected day
in the calendar. Whenever the selected date changes, the class is
added to the corresponding button and removed from all others.
However when the selected date doesn't change (i.e. when clicking
the already selected date), the buttons are not updated and the
use of the :active class conflicts with StButton's builtin handling
of the class - the class is removed on the button up event and the
button is deselected.
Fix this by simply using a different pseudo class.

https://bugzilla.gnome.org/show_bug.cgi?id=746867