Update FlexPanel default property values to match FlexBox specification

[Al12rs]

Problem:

Some of the properties of FlexPanel have initial values that don't match the official specification of FlexBox.
Specification

This caused default behavior of the FlexPanel to be inconsistent with FlexBox behavior, which it is supposed to be emulating.

Changes:

• AlignItems property:
  • old: AlignItems.FlexStart
  • new: AlignItems.Stretch
• AlignContents property:
  • old: AlignContent.FlexStart
  • new: AlignContent.Stretch
• Wrap property:
  • old: FlexWrap.Wrap
  • new: FlexWrap.NoWrap

All new values were taken from the official FlexBox Specification.

Impact:

This is a breaking change with respect to the default behavior of the FlexPanel and is likely to cause a disruption to current users updating to the new version.

To fix any unwanted changes in behavior, each usage of FlexPanel would have to be checked and updates be made to overrule the new default property values with the old ones.

Since this would be quite a tedious process, the following styles are provided as an alternative to revert the initial values to what they were before this PR.

<!-- Override FlexPanel initial values to revert https://github.com/AvaloniaUI/Avalonia.Labs/pull/74 -->
<Style Selector="panels|FlexPanel">
  <Setter Property="AlignItems" Value="FlexStart" />
  <Setter Property="AlignContent" Value="FlexStart" />
  <Setter Property="Wrap" Value="Wrap" />
</Style>

While this change will cause disruption in the short term, it should prove a benefit in the long run.
This is specifically useful to developers adapting UI designs into Avalonia from HTML/CSS based sources, like Figma or other deign tools or existing UIs.

Having different defaults from FlexBox results in confusion and frustrations for the developers.

Since the whole point of this Avalonia.Labs repository is to host controls and panels that are not API stable, it feels important to make this kind of changes now.

[maxkatz6]

It's a breaking change, but better to do it early than too late. While this panel isn't actively used.