AppleTV support + improvements for iOS backend

[maxkatz6]

What does the pull request do?

1. Avalonia.iOS backend cleanup, conditionally disabling some features on unsupported platforms (respecting both tvOS and macCatalyst).
2. Keyboard and remote-control keys/gestures handling.
3. Initial Metal support for iOS backend was added. It only works well on iOS, as tvOS and macCatalyst require SkiaSharp 3.0. Disabled by default for now, as it's not stable.
4. Enabled Nullable and various warnings fixes.
5. Minor changes on top of that, like improving Clipboard support for a bit.

How was the solution implemented (if it's not obvious)?

For the most part, Avalonia.iOS backend is reused, as it is compatible with tvOS.

The same would go with macCatalyst, but: it doesn't support EaGL, and SkiaSharp 2.88 seems to have a broken Metal support for macCatalyst, so no go here. For now.

Fixed issues

Fixed #12523

Improtant

Currently it works well in simulator, but on real device we had a blocker: mono/SkiaSharp#2715

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0043586-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0043783-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0043789-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[kekekeks]

LGTM in general, a few minor comments