Codechange: Make all NWidgetPart arrays constexpr.

This ensures that the arrays are not created at runtime and prevents using non-constexpr values.

src/main_gui.cpp

@@ -176,7 +176,7 @@ void FixTitleGameZoom(int zoom_adjust)
 	vp->virtual_height = ScaleByZoom(vp->height, vp->zoom);
 }
 
-static const NWidgetPart _nested_main_window_widgets[] = {
+static constexpr NWidgetPart _nested_main_window_widgets[] = {
 	NWidget(NWID_VIEWPORT, INVALID_COLOUR, WID_M_VIEWPORT), SetResize(1, 1),
 };