refactor mobiledoc to get rid of controller

[alidcast]

Ideally a package will only consist of an Editor and its nested, complementary components.

[alidcast]

Since button component is functional, and functional components are rendered before slots, it can't receive provided props from the editor, only from root instance (see vuejs/vue#5837). Thus, we need a shared vm/controller that both editor and its nested components share.

[alidcast]

update: separate controller seems to be necessary since runtime build does not compile templates, which means that you can't use it as both a template and as a controller (and vue files are too limited)