Make Add/Remove Logical public

[PureWeen]

Description of Change

~Microsoft.Maui.Controls.Element.AddLogicalChild(Microsoft.Maui.Controls.Element element) -> void
~Microsoft.Maui.Controls.Element.InsertLogicalChild(int index, Microsoft.Maui.Controls.Element element) -> void
~Microsoft.Maui.Controls.Element.RemoveLogicalChild(Microsoft.Maui.Controls.Element element) -> bool
Microsoft.Maui.Controls.Element.ClearLogicalChildren() -> void

• Adds a set of APIs (AddLogicalChild, RemoveLogicalChild, ClearLogicalChildren, InsertLogicalChild) to streamline our setup of parent/child relationships and also allow for outside developers to opt into the benefits of becoming a logical child.

• Expose a set of public APIs on Element that allow users to add/remove logical children to any Element. There are a number of features we have that rely on LogicalChildren: LiveVisualTree, HotReload, Style propagation, etc. The LogicalChildren structure is currently closed off from the outside world. If someone implements a custom control based off of Element, there's not really an easy way for them to setup LogicalChildren. This makes it almost impossible for 3rd party controls to setup a proper hierarchy that will benefit from LiveVisualTree, HotReload, Style propagation, BC propagation, etc.

• Rework internal code to use these new APIs

Examples

For example, the MCT has a popup control that lives on a Page. Because that popup can't add itself to the LogicalChidlren of a Page the popup control doesn't work with Hot Reload or Live Visual Tree

CommunityToolkit/Maui#1168

var page = new ContentPage()
{
	Content = new VerticalStackLayout()
};

var customControl = new MyCustomControl();
page.AddLogicalChild(customControl);

fixes #3745
fixes #15760

[PureWeen]

This PR adds a slightly longer delay to OnLoaded this gives Android a little more time which causes an OnMeasure loop to now fire on the ContentPage before these tests start running. Because of this, Android also needs this wait.

[mattleibow]

This is looking really good - makes me so happy to remove all the internal magic!

[MartyIX]

Suggested change

	/// Removes all <see cref="Element"/>s.
	/// Removes all <see cref="Element"/>s.

[MartyIX]

Suggested change

	Assert.True(e.ChildIndex >= 0,$"Visual Tree inaccurate when OnVisualTreeChanged called. ChildIndex: {e.ChildIndex}");
	Assert.True(e.ChildIndex >= 0, $"Visual Tree inaccurate when OnVisualTreeChanged called. ChildIndex: {e.ChildIndex}");