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/libs/cplusplus/PPToken.h

@@ -129,10 +129,10 @@ public:
     { return m_src.constData(); }
 
     const char *tokenStart() const
-    { return bufferStart() + offset; }
+    { return bufferStart() + byteOffset; }
 
     ByteArrayRef asByteArrayRef() const
-    { return ByteArrayRef(&m_src, offset, length()); }
+    { return ByteArrayRef(&m_src, byteOffset, bytes()); }
 
 private:
     QByteArray m_src;