Hidden xray-vision dependency is still not dealt with anywhere

[flowln]

There's still a xray-vision hidden dependency in the LiveImage callback, mentioned in previous issues (#371 #823). This dependency is not mentioned anywhere in the README, the docs, or the requirements, so you only encounter it if you're reading the code, or if you happily stumble upon a nice ImportError on your bluesky journey 🙂

Expected Behavior

No error should be raised on normal operation, if you install a package directly from where it is made available oficially.

Possible Solution

1. Vendor-in the relevant code from xray-vision into Bluesky.
2. Add the xray-vision dependency to the project requirements (possibly optional, like the matplotlib dependency, though it should not make the callback accessible when it is not installed).

[callumforrester]

Reviewers: @tacaswell (who thinks vendoring in is the better path)

[stan-dot]

here is the relevant file to vendor-in. To make it work likely the whole package will need to be imported

[tacaswell]

The abstract layers were a bit of architecture astronaut exercise. We only ever ended up with one actual implementation so in the same move we should also squash down what ever it is we are inheriting.

[stan-dot]

The mpl_package supports three kinds of display: CrossSection, Stack1d and Contour. We only use the first one. Should all the others be deleted?
@tacaswell

[tacaswell]

I would phrase it as "only port what we need", but yes.