Give shortcut overlay a distinctive opening animation (to imply it isn't an app)

## Problem

I feel like it's good you can't drag it around, but it _still_ doesn't feel as system component-y like the audio and brightness overlay or applications menu.

## Proposal

Hardcode/find a smart way for Gala to animate the opening of the overlay differently, maybe the window could:
- **Classic fade in/out** (the same way notifications fade out)
- **Fade in/out but subtly play a slight 'flip' animation when opening** (a little whimsy, but it might be funny to try and see how well it works outside of notif's)
- **Fade in and slide in from the bottom¹** (could make it resemble getting out a map)
- **Fade in and slide in from the top¹** (could look like it's coming from wingpanel, an unchanging system component)
- **Reverse app opening anim by shrinking in rather than expanding**
¹ Play the last X milliseconds of the slide in or out like the application menu does, otherwise it could be nauseating

## Prior Art

Applications menu, notifications, wingpanel and other system components do already have their own animations

Compare the shortcut overlay with the existing animations for the applications menu, notifications, wingpanel, and other system components. The proposal lists several possible animation styles but does not select one; the work is done only after a specific direction is agreed and the overlay clearly feels like a system component.

Written by the indexing model from the issue text.