New logo

The logo we have was kind of made in a hurry (took a couple hours) early on just so we'd have something. (Discussion in https://github.com/ActivityWatch/activitywatch/issues/9)

I've thought about this a bit, and have some ideas for an improvement.

Specifically:

- I want to get rid of the "watch" component, the name is confusing enough
- Edit: or maybe not? It doesn't really bother me anymore.
- I want a little (but not too much) color, definitely less white
- I think a sunburst would look pretty nice

I'm not going to hurry this like last time, but wanted to get this down.

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Start by reading the discussion linked from issue #9 and the design ideas recorded here. Confirm the intended direction with maintainers before working; done means an agreed replacement logo that addresses the unresolved watch component, color, and sunburst preferences.

索引モデルが issue の本文から書いたものです。