dateMenu: Allow extensions to add custom calendar sources

Splits instantiation of the event source into a separate method,
allowing extensions to subclass the DateMenuButton and provide its
own calendar source.

https://bugzilla.gnome.org/show_bug.cgi?id=672500
Calendar: Remove leading zeros for 24h time format

https://bugzilla.gnome.org/show_bug.cgi?id=658675