Fix changing the number of rows via the dbus protocol

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