Improve AppDelegate

[PoignardAzur]

Some things to improve:

• Multiple windows. Right now AppDelegate assumes there's only one window, and ignores all except the first one.
• Sending commands. Right now the AppDelegateCtx type doesn't have a method to send commands. That should be fixed (though it should be part of a large strategy, see #1)
• Implement for FnMut. If we expect most AppDelegate implementations to only implement a single method, we may just want to implement it for an FnMut taking the same parameters as that method.
• Maybe skip downcast. She should have enough type information to know the widget type of at least the first window. We might want to use it to skip a downcast.