Implement windowWillUseStandardFrame

Seems that `NSWindow.isZoomed` returns the wrong result unless you implement this? ¯\_(ツ)_/¯

native/Avalonia.Native/src/OSX/AvnWindow.mm

@@ -223,6 +223,19 @@
     }
 }
 
+// From chromium:
+//
+// > The delegate or the window class should implement this method so that
+// > -[NSWindow isZoomed] can be then determined by whether or not the current
+// > window frame is equal to the zoomed frame.
+//
+// If we don't implement this, then isZoomed always returns true for a non-
+// resizable window ¯\_(ツ)_/¯
+- (NSRect)windowWillUseStandardFrame:(NSWindow*)window
+                        defaultFrame:(NSRect)newFrame {
+  return newFrame;
+}
+
 -(BOOL)canBecomeKeyWindow
 {
     if(_canBecomeKeyWindow)