fix Caption::size()

Also include margin space in computed size.

src/caption.cpp

@@ -90,6 +90,8 @@ abstract::Widget::Size widget::Caption::size() const
     abstract::Widget::Size size;
     TTF_SizeUTF8(_font, _text.c_str(), &size.w, &size.h);
 
+    size = {size.w + margins.w, size.h + margins.h};
+
     return size;
 }