[LiveComponent] Smooth out parent-child component relationship

[weaverryan]

Q	A
Bug fix?	yes
New feature?	yes
Tickets	#102 addresses Bug C and Bug F
License	MIT

This PR is to address a few outstanding issues related to nested or "parent/child" components:

• [LiveComponent] Meta issue for todos, changes & bugs #102 Bug C: When a model is updated in a child component, that parent component does not receive that component update (e.g. imagine a form that renders a component... and one field is rendered inside another component - when the child component's model is updated, it should also update that same model in the parent component [the form]).
• Change data-model to take priority over name so that child components can use data-model and allow parent components to still use the name attribute to match up with their model.
• Change the new live:update-model event to also (when possible) pass the relevant element so that the listener can look at both the name and data-model attribute to find a match. Or find an alternative solution.
• Document how parent/child components work (e.g. that when a parent re-renders, the child does not re-render)
• [LiveComponent] Meta issue for todos, changes & bugs #102 Bug F: Add a way to "force" a child component to re-render
• Document a known edge case: if a child component re-renders, and the shared model value (shared with a parent component) CHANGES in the AJAX response (e.g. content=foo is sent to the server but content=FOO is returned, and so FOO is now the new value of the content model), a parent will not be aware of this change.
• Add way to "map" a child model onto a parent model so that you can "map" a private LiveProp from a child onto a parent so that it's not lost if the parent renders over the child.
• Standardize / cleanup more tests with mockRerender().

[weaverryan]

This is ready to go - the README.md is updated. Mostly, this makes parent-child relationships much smarter to capture the "95%" use-case without any extra config.

[kbond]

Reviewed the readme: looks great. The full example is a nice touch!