cubx-accordion-ctrl: allow preselect

[jtrs]

Give a configuration for preselect by initalisation.

Add expanded Property to itemSelectors structure:

<cubx-accordion-ctrl cubx-webpackage-id="this">
    <cubx-core-init style="display:none;">
        <cubx-core-slot-init slot="itemSelectors">
            [
            {"head": ".accordion-head11", "body":".accordion-body11", "expanded": true},
            {"head": ".accordion-head12", "body":".accordion-body12"},
            {"head": ".accordion-head13", "body":".accordion-body13", "expanded": true},
            {"head": ".accordion-head14", "body":".accordion-body14"}
            ]
        </cubx-core-slot-init>
    </cubx-core-init>
</cubx-accordion-ctrl>