Apparently it's not allowed to set root context properties multiple
times. If one goes to systemsettings, opens effects kcm, closes it
and opens it again it crashes due to setting a context property with
same name again.

This change eliminates the need for the context property by modifying
the property of the QML objects directly.

BUG: 354164
BUG: 351763
FIXED-IN: 5.4.3
REVIEW: 125737