Avoid connecting to screen changed signals twice

Summary:
load() can be called multiple times; either from setShell or
loadLookAndFeelDefaultLayout. We still only want addOutput once when a
screen is added

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3320

CCBUG:372099
CCBUG:371858
CBUG:371991
CCBUG:371819
CCBUG:371734