App: add Property::isSame() API

To compare if two property contains the same content. The default
implementation in Property uses the persistense interface to save both
properties to string and compares the content. This may not work at the
moment if the property saves content in separate file or in binary.

Various properties have cheaper implementation to direct compare their
internal values.

src/App/PropertyStandard.cpp

@@ -207,7 +207,7 @@ void PropertyPath::setValue(const char * Path)
     hasSetValue();
 }
 
-boost::filesystem::path PropertyPath::getValue() const
+const boost::filesystem::path &PropertyPath::getValue(void) const
 {
     return _cValue;
 }