Introduce QTextDocument::baseUrl

Required for QQuickText & friends to implement image resource handling
in a clean way - to get rid of QQuickTextDocumentWithImageResources and
eventually make QQuickTextEdit's document interchangeable.

Change-Id: I97314a6c3e2d5726539d5722795a472631388cb0
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>