[KFileMetaData] Add extractor for generic XML and SVG

Summary:
Currently, both XML and SVG documents are indexed as plain text due
to mimetype inheritance. This fills the content index with meaningless
data (tags, attributes, attribute values ...).

Use QDomElement::text() for generic XML documents and <text/> nodes
for SVG to extract the content. Also try do find Dublin Core metadata
and add the relevant properties.

Depends on D16488

Reviewers: Frameworks, astippich

Reviewed By: astippich

Subscribers: kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16489