[desktop_multi_window] Proposal: Add a `resizable` boolean setter to `WindowController`

It would be nice to be able to control whether windows are allowed to be resizable or not.

Example:
```dart
windowController
  ..setFrame(const Offset(0, 0) & const Size(350, 350))
  ..center()
  ..setTitle('About {{app_name}}')
  ..isResizable(false) // This is what I'm suggesting
  ..show();
```

