Added WrapPanel item alignment

[TomEdwardsEnscape]

Adds WrapPanel.ItemAlignment, which allows wrapped items to be left/top, centre, or right/bottom aligned.

The property is an enum type with the values are Start, Center, and End. These names encompass both horizontal and vertical orientations.

What is the current behavior?

Items are always laid out starting from the left/top of a WrapPanel. There is no way to configure this.

Breaking changes

None.

Obsoletions / Deprecations

None.

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054001-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[robloo]

I think we should debate the API here. It doesn't really match with HorizontalAlignment, HorizontalContentAlignment or TextAlignment.

[TomEdwardsEnscape]

This is because the panel can also be oriented vertically too. A unified alignment property like the one in this PR is also used by the CSS flex-box.

[timunie]

Yeah the names are okay to me. However I would like to align the new enums, given the team accept this PR, with Avalonia.Labs VirtualizingWrapPanel, which I have a draft PR open.

In addition to start, end and center, should we add Stretch? Also uniform distribution may be nice. Not sure if the later needs to be part of this PR.

[stevemonaco]

Keep in mind that FlexPanel exists in Avalonia.Labs which has a lot of overlapping functionality. See screenshot:

Some decision probably needs to be made regarding whether WrapPanel is a layout control specifically intended for WPF ports, possibly with spacing added to match WCT's UWP/WinUI3 implementation, or whether it starts to march towards a flex box.