[changed] Replace appElement with getAppElement

This makes getAppElement a required prop as well as makes
it a function that will be called expecting a DOMElement.

closes #287

This also takes some inspiration from #359 for handling arrays
of objects.

Upgrade Path:
  - If you had specified an appElement via `Modal.setAppElement`,
    then you need to convert that to a getAppElement prop on the
    modal, this should be a function that returns either a single
    element or an array of elements.
  - If you had nothing specified you will need to add the getAppElement
    element to prevent beakages.