Moved TextEditDocumentLayout and related classes to their own file

The Parentheses, TextBlockUserData and TextEditDocumentLayout classes
and their member function implementations were spread around the
BaseTextEditor class. Moving them to their own file to make the code a
bit better organized.

Reviewed-by: mae

src/plugins/cpptools/cppmodelmanager.cpp

@@ -85,6 +85,9 @@
 #include <QtCore/QTime>
 #include <QtCore/QTimer>
 #include <QtConcurrentMap>
+
+#include <QtGui/QTextBlock>
+
 #include <iostream>
 #include <sstream>