Granite.Revealer

### Problem

Gtk.Revealer has animations that don’t feel that great and by default it clips shadows. Plus it’s missing animation modes like scale

### Proposal

A widget Granite.Revealer with some nice canned animations

### Prior Art (Optional)

We wrote some really great custom animations with fun easings for the Dock that it would be nice to re-use easily in other apps. The one for apps to slide in from the bottom of the Dock could be adapted to reveal from any direction. The scale in animation for badges would be great to have for other kinds of overlays

Start by reviewing Gtk.Revealer and the prior animation work from the Dock described in the issue. Define what Granite.Revealer should provide, including canned direction and scale animations, and consider the work complete when the proposed widget offers the requested reusable animations without the stated clipping limitation.

Written by the indexing model from the issue text.