Help: Only open 'internal' documentation links in Qt Creator

Open external webpages in external browser.

Task-number: QTCREATORBUG-8433
Change-Id: I760f6c5705046a6f8b7ebec3cd7b32253738e0df
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>