Pick up on virtual desktop name changes at runtime.

Summary:
Includes a fix for refresh() being called too many times because
UniqueConnection doesn't work.

However, this revealed that KWindowSystem::desktopNamesChanged
is emitted many times, which Martin should take a look at.

CCBUG:368579

Reviewers: #plasma, davidedmundson, graesslin

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2806