Reload virtual desktop name on change

Summary:
PlasmaVirtualDesktop at a protocol level is double buffered.

There's a done() command emitted after property changes. After this the
name might have changed.

Test Plan:
Changed a desktop name (with relevant kwin patch)
Name changed in the pager tooltip

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: zzag, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18512