Fix crash when spliting GLSL editor

Fix crash when spliting GLSL editor

Not a good idea to keep the editor stored since when removing
a split, for example, the editor will go away and the highlighter
is not aware of it anyhow since it actually belongs to the document.

Although it's not necessary for the fix this patch add another
constructor to SyntaxHighlighter which takes the BaseTextDocument.
This is convenient.

Task-number: QTCREATORBUG-5695
Change-Id: Ic91837b7d91ebd3a44c16e2fd589d7f6c5c0c002
Reviewed-on: http://codereview.qt.nokia.com/2508
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>