Evolution should be able to subscribe to calendars on the local file system
(like KDE Kontact).

Usage scenario:
A small office wants to share calendars easily (without setting up a CalDAV
server, for example). The office already has a network file system (such as NFS
or CIFS), so workers place their calendars in a common directory.

Implementation suggestions:
* The relevant dialog has a browse button to make finding the file easy.
* The user is able to drag an .ics file from Nautilus onto Evolution, and then
Evolution asks, "Would you like to import this file or would you like to
subscribe to it?"