demo browser: Fix the compilation with QtWebEngine.

This makes the demo compile, but not link since
most of the methods aren't implemented yet.

Also disable downloads and printing since they require a bit
more work to instead use the page directly.

Change-Id: I59adfe07fda077c6909f70f12800a4cfa6a6dad2
Reviewed-by: Andras Becsi <andras.becsi@digia.com>