DAV resource: propagate color changes to the server

Summary:
The resource didn't implement collectionChanged at all.

Now it does; saving colors work. I realized this meant renaming
folders didn't work either. But that's for another commit because
there are dragons due to the tree structure being flattened.

Depends on D28937 to actually trigger the collection modify job.

Test Plan:
Change color in korganizer, refresh roundcube, it shows the new color.

What I'm not sure about is what "Disable color" is supposed to do.
The code says "save an invalid color". OK... but what does it mean for the user?
Right now the calendar just gets some random color instead. What purpose does this serve?
Why not remove this action, which would remove a submenu i.e. improve usability?

Reviewers: dvratil, ochurlaud, vkrause, winterz, mlaurent

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28938