Add events for widget is mounted and unmounted from UI tree

[eruvanos]

Enhancement request:

What should be added/changed?

Widgets should be able to know, when they are added or removed to the ui.

What would it help with?

This would be useful for widgets like UIDropdown or any other widget, that requires access to the UIManager

Options

I see two options in general:

A) widgets receive these events if they are mounted or removed to the uimanager itself or any of a connected child. B) widgets receive these events if they are added or removed to any parent (this would not provide the requested improvements I think)