Ensure _NET_CURRENT_DESKTOP is set on startup

Summary:
Seems to have regressed in Plasma 5.12 due to code reordering. Now the
property is explicitly updated once the NETRootInfo is created.

BUG: 391034
FIXED-IN: 5.12.5

Test Plan:
Test case exposing the problem added. Fails without the patch,
succeeds with the patch.

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10836