C++: Clarify units of a Token

This will avoid confusion when later more length and indices methods are
added.

In Token:
    length() --> bytes()
    begin() --> bytesBegin()
    end() --> bytesEnd()

Change-Id: I244c69b022e239ee762b4114559e707f93ff344f
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cppeditor/cppeditor.cpp

@@ -794,7 +794,7 @@ void CPPEditorWidget::markSymbolsNow()
             if (column)
                 --column;  // adjust the column position.
 
-            const int len = unit->tokenAt(index).length();
+            const int len = unit->tokenAt(index).bytes();
 
             QTextCursor cursor(document()->findBlockByNumber(line - 1));
             cursor.setPosition(cursor.position() + column);