Assistant does not display documentation correctly when built without webkit

Reported by several users, including meego project: http://bugs.meego.com/show_bug.cgi?id=5959

When Qt is built without Qt WebKit, the fallback is QTextBrowser, which does not display the documentation correctly. In the case where Qt WebKit is shipped separately, it makes sense Qt Assistant detects Qt WebKit at runtime, not build time. Another solution is to fix QTextBrowser to display the documentation properly.