`windows-reactor` window size API improvements

Suggestion

It would be great to be able to specify an initial x and y position + is_maximized in WindowVisuals

plus, for on_window_size to notify is_maximized (it will emit when the maximize button is clicked, however it emits as just a regular size change)

plus, a on_window_position to notify of the x and y changes

In my app, I store these properties (along-side width and height) to restore window sizing for the user on app reopen