C++: handle file type changes for choosing highlighters.

If the file type (mime type) of an editor changes, it might need a
different kind of highlighter/highlighting-support.

Change-Id: I470dbf69e71856c9593d201416c4d4bd2958aaec
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/texteditor/basetextdocument.h

@@ -105,6 +105,7 @@ public:
 
 signals:
     void titleChanged(QString title);
+    void mimeTypeChanged();
 
 private:
     void cleanWhitespace(QTextCursor &cursor, bool cleanIndentation, bool inEntireDocument);