Keep _NET_DESKTOP_NAMES in sync

Summary:
We have to connect to nameChanged even if m_rootInfo is not set yet,
otherwise names in _NET_DESKTOP_NAMES won't be kept in sync if
a virtual desktop is renamed.

CCBUG: 403307

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18503