Don't allow an unrecognized child element to block loading of a shape plugin.

Summary:
If an object element has child elements but no shape plugin can be resolved for any of them fall back to trying to create a plugin from the root element.

This guards against cases like the snippet below where an experimental tag has been embedded in an element which would have otherwise correctly loaded the charts plugin.

<draw:object draw:notify-on-update-of-ranges="Лист1.B2:Лист1.B12" xlink:href="./Object 1" xlink:type="simple" xlink:show="embed" xlink:actuate="onLoad">

<loext:p />

</draw:object>

Reviewers: pvuorela

Reviewed By: pvuorela

Subscribers: Calligra-Devel-list

Tags: #calligra:_3.0

Differential Revision: https://phabricator.kde.org/D25031