[libtaskmanager] Fix shared VirtualDesktopInfo::Private tied to the 1st creator

Summary:
VirtualDesktopInfo::Private is ref counted and shared between N
VirtualDesktopInfo's.

We were passing the first VirtualDesktopInfo as a parent to
VirtualDesktopInfo::Private which didn't manage the lifespan or do
anything useful.

The wayland backend incorrectly used this 'q' object and spanned
mulitple internal objects and connections to the first object that
created the shared private instance, leaving us with dangly objects
inside.

BUG: 415200
(and possibly several other bugs of wayland task manager not updating
correctly)

Test Plan:
Compiles
Plasma still loads

Reviewers: #plasma, zzag

Reviewed By: #plasma, zzag

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27266