[AppletQuickItem] Only set QtQuick Controls 1 style once per engine

The style is global per engine, there's no need to set it for every applet created
as the engines are shared.
Since this entire thing is just a hack, just setting a dynamic property to identify
that we've set a style is valid imho.
Also turn it into a plain QtObject since we don't need a fully-fledged Item.

Differential Revision: https://phabricator.kde.org/D4362