Obsolete setting mainpage via application

[PureWeen]

Description of Change

• Obsolete Application.Current.MainPage (set/get)
• change all templates to use the override CreateWindow vs setting the MainPage on Application
• Provide guidance (docs and obsolete message) indicating how users can achieve the same thing
  • Application.Current.MainPage can become Application.Current.Windows[0].Page
  • If you're on an element that's attached to the window you can do "this.Window"
• Make the following extension method public so developers can easily get to the IWindow from the platform UIWindow. Android and WinUI already have public APIs for this

  maui/src/Core/src/Platform/iOS/UIApplicationExtensions.cs

  Line 77 in fe224da

  public static IWindow? GetWindow(this UIWindow? platformWindow)

  .
• Create Item Template for a Window

Issues Fixed

Fixes #23518

[Eilon]

Thinking out loud - this will cause a warning (or possibly error) in 100% of MAUI applications ever written, no? Is it that bad to keep this? Maybe an analyzer could be used to flag it and rewrite the code for those interested? (And ignored by those who don't.)

[PureWeen]

Yea, I just worry about also dragging the past along with us and not overall widening the pit of success

From a library author perspective using these APIs is really bad. For example, none of this code from our toolkit will work for multi-window https://github.com/CommunityToolkit/Maui/blob/cc1fb686815587076bbad45a1903f3ec3039315d/src/CommunityToolkit.Maui/PopupService.cs#L17

We probably see an issue a month for android where users need to override CreateWindow

#23645
#23493

I also worry about new users using MainPage and ignoring the analyzer suggestions.

The fix here is basically to do a solution search for
MainPage and replace it with Windows[0].Page

I get it because how widespread it is that users will all see warnings, but, it just seems to put MAUI in general on a path to longer term refinement/success.

I could see us maybe Obsoleting the set and just using an analyzer on the get perhaps?

[MartyIX]

Existing non-trivial apps already have various workarounds. There are still various gotchas in APIs. It's been getting better but it's not like there are no sharp edges.

Given that MAUI is relatively young, personally, I would be more radical, especially if it can lead to people not hitting bugs in the future (which translates to not losing more resources).

Analyzer might be nice in the sense: fix it for me fast.