Update app icon

Hi! I have noticed that the current app icon looks a little out-of-place among other GNOME app icons, as the clock seems to be "standing" instead of "lying down" in the app grid like other icons, and the aspect ratio of it makes it only cover about half of the canvas, which looks a little awkward. Also, the app has a pretty cool, distinctive UI, and I wondered if that could be reflected in the icon somehow. Based on this, I have:

• "flipped" the clock 90° backwards
• reduced the chin size down to the recommended 2 detail units
• tweaked the aspect ratio to avoid the icon being awkwardly short in this orientation
• divided the icon surface into two distinct shades of brown like in the app.
• generated a devel version for nightly versions. Actually using this requires some code changes, I don't know if it's really needed for this app, but added it anyways.

I haven't changed or added any elements except from this, the current icon was pretty easy to work with 😃

In addition to that, I have tweaked the symbolic slightly to make the difference in button height seem less jarring, and generated a project avatar using Emblem. Potential use cases include the project's GitLab logo, a Matrix room avatar, social icons, etc.

I hope this is helpful in some way! 😄