Fully reinitialize graphics window in AfterNewFile().

Before this commit it was possible to cause a crash by e.g. selecting
a face and pressing Ctrl+N, or numerous other glitches.

src/graphicswin.cpp

@@ -242,7 +242,7 @@ void GraphicsWindow::Init(void) {
     context.active = false;
 
     // Do this last, so that all the menus get updated correctly.
-    EnsureValidActives();
+    ClearSuper();
 }
 
 void GraphicsWindow::AnimateOntoWorkplane(void) {