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.cpp

@@ -55,8 +55,8 @@ int ByteArrayRef::count(char ch) const
 void Internal::PPToken::squeezeSource()
 {
     if (hasSource()) {
-        m_src = m_src.mid(offset, f.length);
+        m_src = m_src.mid(byteOffset, f.bytes);
         m_src.squeeze();
-        offset = 0;
+        byteOffset = 0;
     }
 }