[nightcolor] Expose transition timings to d-bus

Summary:
These timings can be useful for the night color applet. For example, it
could show "Next transition will start at XXX" or something like that.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26666