To keep the current date chooser in sync with
GNOME Shell, tweak the week names to show only
the first, upcased letter of the week name.
Also, this commit avoids making day number circles
ellipses when the week number is not present.
This is mainly a copy-paste from an old branch authored
by Matthias Clasen. The code style was updated to match
Calendar's one, and the theming was improved a little bit.