QQuickTextDocumentWithImageResources: use QTextDocument::baseUrl

QTextDocument::baseUrl was introduced in qtbase commit 1f22c1d. The
ultimate goal is to get rid of QQuickTextDocumentWithImageResources
and eventually make QQuickTextEdit's document interchangeable.

Change-Id: Ibafeecc395b52f7200d8164f809bf76538e5620c
Reviewed-by: Alan Alpert <aalpert@blackberry.com>