[kcmeffects] Do not use root context properties

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