feat(NumericUpDown): Add InnerLeftContent and InnerRightContent

[workgroupengineering]

Add InnerLeftContent and InnerRightContent to NumericUpDown.

What does the pull request do?

What is the current behavior?

What is the updated/expected behavior with this PR?

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

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

Fixes #14314

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0044757-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[stevemonaco]

Is there some reasoning as to why only InnerContent and not both InnerLeftContent and InnerRightContent as TextBox has? Besides visually crowding the spinner side.

I have a reworked version of NumericUpDown in a local project that uses both. Left side is a label and the right side is a button. Either way, my customization goes further beyond so it wouldn't save me a fork.

[jp2masa]

Another common scenario is to add a units label. When the spinner buttons are placed on the right of the text box, this label would be placed between them.

[maxkatz6]

Yes, inner content on the same side as buttons is a common scenario. Or even on both sides - icon on left, and units on right.

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0044816-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]