Change lifecycle

[tomalec]

Currently we do all the work (importing, fetching and stamping elements, attaching the model) on attachedCallback.

What if we:

1. Import nodes on createdCallback and every content attributeChangeCallback even when detached
2. Attach model on createdCallback, every model attributeChangeCallback, once new content is imported, before it's (re-)stamped.
3. Attach nodes after element is atached
4. Detach nodes once element is detached (keeping the reference)

@warpech @miyconst WDYT?

Benefits:

• we could start importing content sooner => improve FOUC
• we could access the imported content before stamping in more gently way (default layouts for juicy-composition)
• we could easier move imported-template-scope magic to separate component, that will plug-in to this

Risks/Doubts:

• imported elements will be created before attached to main document, so if they well created with such assumption they mail fail (however it is a bad practice anyway)
• flow may seem more tangled (however, I believe that with proper explanation in may be easy to get)