Show week numbers in month view header

Currently, month view is the only view in gnome-calendar where you cannot see any info about week numbers. For reference, week view shows the week number in the header, and year view shows the week numbers at the beginning of each week's line, if you have activated the show-weekdate gsetting, either using gnome-tweaks or by gsettings set org.gnome.desktop.calendar show-weekdate true.

This change adds a "weeks X - Y" label to the heading of the month view, similar to the "week X" label in week view.

Might be part of a fix to #378 (closed).