Switch (back) to our own Color type instead of using wxColour.

The reason is that wxColour's default constructor creates an invalid color (what is that even?). It is nicer to just have default be transparent.

src/gui/value/color.cpp

@@ -151,6 +151,6 @@ void ColorValueEditor::change(const Defaultable<Color>& c) {
   addAction(value_action(valueP(), c));
 }
 void ColorValueEditor::changeCustom() {
-  Color c = wxGetColourFromUser(0, value().value());
-  if (c.Ok()) change(c);
+  wxColour c = wxGetColourFromUser(0, value().value());
+  if (c.Ok()) change(Color(c));
 }