Fix document appearing as it is all changed on open

Broken by 113c5600c909791ddfea3d424a41d982dcdda6b7

Apparently SyntaxHighlighter::rehighlight is called too early

Task-number: QTCREATORBUG-10026
Change-Id: Ifa76c156b5b84b79c869d2de1a0ad69c13ac6b48
Reviewed-by: David Schulz <david.schulz@digia.com>