avoid duplicate desktopviews on first startup
Summary:
on first startup all the desktop views already exist after the
script has been executed, so check the screen doesn't already have
a view for that screen before creating one
BUG:368891
Test Plan: tested both first and subsequent startups, one desktopview per screen created, containment config dialog coming up (and only one of them)
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D2885