Give Pen an out-of-line destructor

src/pen.cpp

@@ -39,3 +39,5 @@ Pen::Pen(const char *colour_opt, int width, wxPenStyle style)
 , colour_con(OPT_SUB(colour_opt, &Pen::OnColourChanged, this))
 {
 }
+
+Pen::~Pen() { }

src/pen.h

@@ -46,4 +46,6 @@ public:
 
 	/// Implicit conversion to wxPen
 	operator wxPen const&() const { return impl; }
+
+	~Pen();
 };