Custom back-button widget

## Problem

The current solution isn't a long term one, and it likely going to look weird to new users who don't see icons inside buttons anywhere else.

## Proposal

Now that libadwaita is a thing, I don't see why Granite shouldn't be able to actually function like a custom library with its own custom widgets. Since flatpak allows apps to bring the elementary stylesheet with them on other platforms the way they look shouldn't be too much of an issue anyway.

## Prior Art

A quick mock-up: