year-view: Read show-weekdate from GNOME Shell

Year view always shows week numbers and by design team the default show
not display them.

This patch adds support to read "show week numbers" setting from GNOME
Shell (org.gnome.shell.calendar::show-weekdate) and to change year-view
accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=763251