Summary:
* save changes to the config files when the layout is saved
* :name() works even if netrootinfo isn't there
* as soon a rootinfo is set, connect all the desktops with name changes

Test Plan:
* tested with the kcm to add, remove and rename desktops, all of that works
* setting the number of rows still only partly works: kwin notices it but
the pager doesn't notice, a plasma restart is needed

Reviewers: #plasma, #kwin, hein, davidedmundson, graesslin

Reviewed By: #plasma, #kwin, hein, davidedmundson, graesslin

Subscribers: zzag, graesslin, davidedmundson, ngraham, kwin

Tags: #kwin

Maniphest Tasks: T4457

Differential Revision: https://phabricator.kde.org/D17265