TextEditor: Disconnect highlighting from text editor content changes

The syntax highligher is connected to the content changes of the text
editor which is not very useful if the diagnostics been provided by
an asynchronous mechanism. So we add a setter to disconnect the changes
from the syntax highlighter.

Change-Id: Ied4b5fe34000b13bafb1aa177b89befe6620d4d7
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/texteditor/syntaxhighlighter.h

@@ -70,6 +70,8 @@ public:
     // Don't call in constructors of derived classes
     virtual void setFontSettings(const TextEditor::FontSettings &fontSettings);
 
+    void setNoAutomaticHighlighting(bool noAutomatic);
+
 public slots:
     void rehighlight();
     void rehighlightBlock(const QTextBlock &block);