[UX+API] Greeter should be a scopes app (showing specific scopes)

[Peter Bittner]

As a developer
I want the Greeter to be a Scopes app
So that I can easily write a Scope ("greeter plugin") that shows data, applies design and styling, and allows for interaction with the notifications or UI elements I want to display.

Background, Motivation, Examples
================================

From a developer point of view we should have all possibilities there are to plug-in into the greeter. Hence, as a developer you should be able to provide an application / plugin / addon (you name it) that provides:

- attaching background images, videos, animations to the greeter
- unlocking via a pin code
- unlocking via gestures (circle, swiping, etc.)
- unlocking via fingerprint scan, iris scan, face recognition, DNA whatever (near-term sci-fi stuff)
- unlocking via remote commands and devices (e.g. via smart watch, near-field communication, etc.)
- placing emergency phone calls (included by default for regulatory reasons, obviously)
- etc.

(NOTE: This is just a list of examples of Scopes (or plugins) that should be able to develop when this feature request is implemented. Also, if this is not feasible with Scopes then some other form of API should be provided to allow easy development of plugins.)

Similar Concepts, Related Bugs, Plugin Ideas
============================================

- Bug 1544670 ("Unique lock screen design w/ circle gestures to unlock")
- Bug 1627747 ("generic API for communication and data exchange")