Make icon set for CalDav event presentation, use in iMIP email.

iMIP e-mails were, as a hack, using filetypes icons for caldav
elements (titles, locations).  This commit creates a folder of
caldav element icons. To start, they are used in iMIP emails,
but eventually should be used by any app that wants to have icon
labels for caldav elements.

Signed-off-by: brad2014 <brad2014@users.noreply.github.com>