Fixing ReactivePage, ReactiveUserControl and DependencyProperties change notification for WinUI 3

[harvinders]

What kind of change does this PR introduce?

The previous PR was not complete, this PR add missing functionality for WinUI

• adds working ReactivePage and ReactiveUserControl,
• WhenAny* now listens to dependency property changes
• ReactiveUI.WinUI library is loaded instead of .net standard version
• Updated xml document for DispatcheQueueScheduler

What might this PR break?
Can't say

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:
ARM64 is still not working, I think it requires .net 6, not sure

[glennawatson]

We still have some other changes we want to get done, like adding net 6 support, but these can wait until after your PR is merged in.

If you got any other stuff you want to PR might be worth adding a checklist to the winui3 issue so we can coordinate on it.

[harvinders]

Yesterday's PR was in error, I wasn't expecting it to be merged so soon. The original PR was based on WinUI preview 3 and since I am bit green with github and forks, I missed couple of things. Following comes to the mind in terms of what could still be added. 1. Tests, but not sure where to start. 2. Adhoc testing with existing app (but that would happen outside the repo) 3. Since, my code was based on the WPF project, few things are added since then. I am now taking the cue from UWP project and few things are still missing, like `WinRTAppDataDriver`, `AutoSuspendHelper` and `SingleWindowDispatcherScheduler`. `SingleWindowDispatcherScheduler` maybe very specific to UWP and we can skip it. `AutoSuspendHelper` might have to wait, as App life cycle is still in preview in Project ReUnion.

…

On Mon, Aug 23, 2021 at 2:57 PM Glenn ***@***.***> wrote: We still have some other changes we want to get done, like adding net 6 support, but these can wait until after your PR is merged in. If you got any other stuff you want to PR might be worth adding a checklist to the winui3 issue so we can coordinate on it. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#2902 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ACBJD5SMBZWCRJZXL43QCHDT6HIKVANCNFSM5CTYH42A> .

[harvinders]

@glennawatson The only issue that I have been able to find is that ObserveOnDispatcher does not work as it resolves to using WPF version in the System.Reactive. It is beyond ReactiveUI and needs to be fixed in there. However, we can still use the alternative ObserveOn(RxApp.MainThreadScheduler). Maybe we can mention it in the release record.

[glennawatson]

We need to probably make a entry for winui3 on the reactiveui.net handbook anyway so will likely be the best location for it.

[glennawatson]

RxApp.MainThreadScheduler is usually the preferred mechanism anyway since it makes unit tests work better.

[harvinders]

@glennawatson Do we have an ETA for the next release?

[github-actions]

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