move the view at the correct index at startup

Summary:
using the same trick as elsewhere, set the currentIndex
and move the view to currentIndex right at startup
the only way to be sure is onContentHeightChanged
as there are no signals for when "the view has been
populated and settled up"

Test Plan:
the view is at the right state since the first frame shown,
no more jumping around effect

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D4703