Use Rust implementation for clipboard

[RunDevelopment]

Fixes #958.

This PR removes the Python clipboard implementation and replaces it with calls into the Rust implementation powered by aboard. As is tradition with Rust, this makes copying images about 2 to 3 times faster on my windows machine. This will hopefully resolve our platform issues. We definitely need to test the new clipboard on our supported platforms before releasing it.

About the new Clipboard API itself: it's pretty simple. It only supports writing text and images, which is enough for us. Clipboard.create_instance() will throw an error if the current platform isn't supported. So we can potentially make a feature for this node if it turns out that aboard don't support some platform.

This is still a draft because I haven't released a new version of chainner_ext with the clipboard API yet.

[joeyballentine]

Are you sure we shouldn't publish the clipboard functionality separately? I'd kinda like to put an actually good clipboard implementation out as its own package since there just isn't one

[RunDevelopment]

We can do that later. Reading numpy images isn't quite trivial, and so clipboard currently shares a bit of code with the rest of chainner_ext. We would have to copy that code into that project as well. A general clipboard library would also need a more extensive API than just being able to write to the clipboard... Right now, I just implemented the bare minimum we need to get rid of the python clipboard code.

[joeyballentine]

I almost wonder if we should publish packages under a namespace or something. Like we could have @chainner/ext and @chainner/clipboard, with the rust repo being a monorepo of all the packages

[RunDevelopment]

1. PyPi doesn't support that AFAIK. The best we can do is sub modules. So e.g. from chainner_ext.clipboard import XYZ.
2. Do you want the independent clipboard package still be tied to chainner? I'd rather make it a separate entity that just lives in the chainner-org organization. Kinda like aboard isn't called @password1/aboard. Unnecessarily associating that package with chainner will just make people think that it is something specific to chainner that can't be used elsewhere.

[joeyballentine]

Yeah I'd rather it be its own package

[RunDevelopment]

Jup, but later. Right now, I want to see whether it actually works.