Unlocking should play a scale effect animation

## Problem

Unlocking right now jarringly just flashes a black screen the moment you unlock, which is disorientating and doesn't utilise the idea that there is a gray 'system layer' beneath everything.

## Proposal

When the user unlocks their device Gala could do a nice fade out of the lockscreen cards and wingpanel, and then a smooth zoom-scale effect of the desktop filling the screen (somewhat like entering a workspace from the overview)

## Prior Art

I rendered out what the transition could look like:

https://user-images.githubusercontent.com/58219504/106654310-e1ebfc80-658f-11eb-9de4-cd2f3673d12e.mov