[WIP] Implement new popovers

No issue

Implements a new popover component (not to be confused with what are now called dropdowns [menus])

Todo:

- [x] Emberify
- [ ] Get checked over by people with Über Ember skilz

Ah, so it is defined!

So, could the same service handle both popovers and dropdowns? Do they need to be separate?

If they do need to be separate services, could you move the `PopoverService`/`DropdownService` object into its own file like `core/client/utils/dropdown-service`, and then both `initializers/dropdown` and `initializers/popover` could import and use it.

You'd then extend it inside of popover if it had specific behavoir