improve meta tag handling

[skurzinz]

Ideally, the dsebaseapp would pull parts of the tei:teiHeader (e.g. all tei:title elements) and output them in html:title and/or html:meta.

Vector:

• add an xql function in modules/app.xql
• call it from templates/page.html

I experimented a bit but could not get the current document's node from within the xql context even though @ingoboerner assisted some. No time for more experimenting, so I'm opening this issue for reference.