Issue title: BackgroundImageDesignable not working
 Issue body: I wanted to go ahead by adding `BackgroundImageDesignable`, but when reading the code, something seems off to me: we create a view, but we don't add / remove it anywhere. So, I tried it (in case I missed something), but it didn't work (normal or bad usage?). Is that expected @phimage ?

Also, while writing the documentation, I started to wondering why is `BackgroundDesignable` necessary? It's just adding an `UIView` for the `BackgroundImageDesignable`, so why not letting `BackgroundImageDesignable` that view itself?

Note for the future PRs, I think (wasn't thinking that before) we should definitely not merge before finalising completely the feature (example, docs, changelog...), otherwise, we are just adding complexity for maintening and have incoherent / incomplete docs / examples. That said, even if it's in the same PR, we can help each other by working on the same branch!
 Comments: 
 Comment 0: The function to add or remove image view is call in `didSet` of image view fields
(like refresh control, that could be too soon if apple do not allow to play with view hierarchy when affecting attributes)

I am testing now on demo app for table cell 
- I go into demo app scene with 4 statics cells (Work, Food,..) 
- remove the ImageViews, children of Content View
- I go to the table cell view and choose a backgroud view
- run the simulator and it seems ok

I will do a test with table too

`BackgroundDesignable` allow to conform to other needs on background view of table and collection views. `BackgroundImageDesignable` is one of them but we could imagine others.

JakeLin comment on it in https://github.com/IBAnimatable/IBAnimatable/pull/416 "It took me a while to understand this..."
 Comment 1: I will test this again following your instruction, I may miss one. Thanks fort he explanation!

It would be interesting to comment the `BackgroundDesignable` since it's not straightforward to understand.
{'source': 'GithubIssues', 'repo_name_with_owner': 'IBAnimatable/IBAnimatable', 'issue_id': 218749336, 'repo_stars': 4002.0}