Add Component.onChildrenChanged() lifecycle method

[st-pasha]

Problem to solve

Currently a component has no way of knowing when it acquires new children or loses existing ones. Which is a shame really, because how it can take good care of its children if it doesn't know what happens to them?

Proposal

Add a new lifecycle method .onChildrenChanged() which would fire whenever new children are added to the component, or existing children removed.

The method could have an argument child, which would indicate the component that caused the lifecycle event. If child.parent == this then the child was just added to the current component; otherwise it was removed.

More information

This would be useful for #1944

[ufrshubham]

Which is a shame really, because how it can take good care of its children if it doesn't know what happens to them?

Quite irresponsible parent they are 😅

[spydon]

@rivella50 you have to write something here for me to be able to assign you to the issue seemingly.

[rivella50]

@spydon Done