Make sure that new virtual desktops can be activated by using plasma virtual desktop protocol

Summary:
We need to connect to PlasmaVirtualDesktopInterface::activateRequested
when a virtual desktop is created, otherwise one won't be able to
activate the desktop by using the pager.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18342