[web] Rename EngineFlutterWindow => EngineSingletonFlutterWindow

[mdebbar]

Make it clear that this class is a singleton.

Part of flutter/flutter#134443

[ditman]

If you want to land this rename, land it, but I think its utility is going to be limited (or short lived).

Take into account that ui.SingletonFlutterWindow is now deprecated:

• engine/lib/ui/window.dart

  Lines 397 to 402 in 10c4803

  	@Deprecated(
  	'Use FlutterView or PlatformDispatcher instead. '
  	'Deprecated to prepare for the upcoming multi-window support. '
  	'This feature was deprecated after v3.7.0-32.0.pre.'
  	)
  	class SingletonFlutterWindow extends FlutterView {

Maybe a better approach here is to make class EngineFlutterView extends ui.FlutterView? (that's probably a much bigger change, because we're going to be missing a lot of getters!)

[mdebbar]

@ditman you are right, this class is going away.

What prompted me to do this rename was your comment. I felt maybe it was not clear that this is a singleton so I wanted to make it clear.

[ditman]

Ahhh, I got you! I think the current name is "correct", except it should eventually extends ui.FlutterView + our web interface rather than the ui.SingletonFlutterWindow, apologies for miscomunicating this (x_x)