Pass message parameters by reference to 2 Partition methods (#788813)

Note that this almost certainly has no performance benefit what so ever
because the methods are implicitly defined as inline [1][2] and the
compiler will have simply inlined the method bodies thus avoiding having
to construct copies of the passed parameters.

Do this anyway as constant objects are typically passed by reference
[3].  Also C++'s std::vector::push_back() [4] takes a const reference
parameter so update the kind of equivalent push_back_message() to take
a const reference parameter too.

[1] C++ reference / inline specifier
    https://en.cppreference.com/w/cpp/language/inline

        A function defined entirely inside a class/struct/union
        definition, whether it's a member function or a non-member
        friend function, is implicitly an inline function.

[2] When should I write the keyword 'inline' for a function/method?
    https://stackoverflow.com/questions/1759300/when-should-i-write-the-keyword-inline-for-a-function-method/1759575#1759575

[3] How to pass objects to functions in C++?
    https://stackoverflow.com/questions/2139224/how-to-pass-objects-to-functions-in-c/2139254#2139254

[4] C++ reference / std::vector::push
    https://en.cppreference.com/w/cpp/container/vector/push_back

        void push_back(const T& value);

Bug 788813 - gparted-0.30.0/include/Partition.h:137]: performance
             problem

include/Partition.h

@@ -133,8 +133,8 @@ public:
 	virtual bool have_messages() const                             { return ! messages.empty(); };
 	virtual std::vector<Glib::ustring> get_messages() const        { return messages; };
 	virtual void clear_messages()                                  { messages.clear(); };
-	void push_back_message( Glib::ustring msg )                    { messages.push_back( msg ); };
-	void append_messages( const std::vector<Glib::ustring> msgs )
+	void push_back_message(const Glib::ustring& msg)               { messages.push_back(msg); };
+	void append_messages(const std::vector<Glib::ustring>& msgs)
 	                                { messages.insert( messages.end(), msgs.begin(), msgs.end() ); }
 
 	// Interface to return reference to the Partition object directly containing the