Fix F1 sometimes opening explorer/finder if documentation is not found

It's not sufficient to try to create a QUrl on a string to find out if
it is an url, because that automatically treats strings without any
special characters as "local file urls".

Task-number: QTCREATORBUG-11570
Change-Id: I18071aed5b3fbdd717b045c2f6e3e90385be8584
Reviewed-by: Eike Ziller <eike.ziller@digia.com>