UI destruction needs to be repeatable

[Reinmar]

As explained in #114 (comment) and in the following comments.

Theoretically, it's simple – if you initialise some components in A, B, C order, then they need to be destroyed in C, B, A order.

So, e.g. if the editor first loads plugins and then inits the UI, the UI needs to be destroyed before the plugins are destroyed.

However, what if one of the plugins created some UI and added it to editor UI's collection? On editor destroy, first the UI will be destroyed (which is recursive and will destroy that plugin's component too) and then the plugins will be destroyed and one of these plugins may want to somehow destroy the component which it created (it might've even initialised it, but usually plugins don't do that).

We've got a collision of responsibilities here. editor.ui.destroy() is recursive so it destroys things which the editor creator didn't create. And then plugins come and try to destroy those things again.

The solution is explained in #114 (comment).

[Reinmar]

Actually, this issue includes two things:

1. Making the UI framework safer to destroy.
2. Reviewing the plugins which it brings because e.g. ContextualBalloon does too much in its destroy() method.