use opacity instead of visible

Summary:
Qt documentation recomends to bind to opacity instead of visible as visible
cascades down. The tabbar used to trigger its height behavior anim when the
popup got hidden, binding to opacity (even if opacity isn't animated) is a
more correct behavior.
Plus with visible it triggers the Qt bug https://bugreports.qt.io/browse/QTBUG-66907
which in turn triggers https://bugs.kde.org/show_bug.cgi?id=391432

BUG:391432

Test Plan: kickoff never opens the last tab anymore

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11098