Summary:
This gives a chance to the resource to store the color server side
(implemented for DAV).

This commit improves upon D7500 (1ec2e08a) which only implemented reading
from the attribute.

Test Plan:
(after implementing saving in the dav resource) changing a calendar color
in korganizer now propagates to the server, F5 in roundcube and the new color appears.
This helps when using multiple computers, or when the dav resource temporarily forgets
about all folder (ahem).

Reviewers: dvratil, ochurlaud

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28937