feat: Intellisense displays only the elements related to the current context

[workgroupengineering]

Allow Intellisense displays only the elements related to the current context

Current behavior

Expected behavior

After PR https://github.com/AvaloniaUI/Avalonia/pull/13104

How does it work

• CompletionEngine get the parent element of the current element, retrieves its type and assigning it to parentType
• If parentType has a property marked with Avalonia.Metadata.ContentAttribute, assigned its return type to parentType .
• Use parentType.ItemsType or parentType to filter candidate types if they are not null

Note ItemsType is set to the type of the parameter of the first non-static public method named Add obtained by traversing the type hierarchy.

Current issue

• If an Item Type has xmlns, xmlns is not entered.

Future enhancement

• Autocompletion of generic properties such as ItemsControl.ItemsPanel
• Autocompletion of ItemsControl derived objects with the correct ItemsType, eg ListBox must propose ListBoxItes and its derivatives.

Depends on PR #383 , https://github.com/AvaloniaUI/Avalonia/pull/13104