[TextBox] Add readonly property: LineCount.

[rabbitism]

What does the pull request do?

Port WPF property to Avalonia
https://github.com/dotnet/wpf/blob/a08bc2f79bb45e46257368363b4bff594a0c848c/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/TextBox.cs#L899-L920

What is the current behavior?

What is the updated/expected behavior with this PR?

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

Some notes about test cases:
The test cases are what I observed from actual running app's behavior.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0053635-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

Not sure how impactful on perf it would be, but LineCount can be a DirectProperty, supporting change notifications.

[rabbitism]

Not sure how impactful on perf it would be, but LineCount can be a DirectProperty, supporting change notifications.

TextLayout.TextLines cannot be subscribed, thus we have to report change for every Text change and Bounds change. It can be expensive.

[MrJul]

I'm really surprised that WPF exposes a non-observable property here.
I can already see the bug reports coming in from bindings not working.

If we can't bind it, maybe it should be a method instead?

[rabbitism]

If we can't bind it, maybe it should be a method instead?

I don't have a preference, please let me know if team get a final decision.