Add a detail column to the Tree widget

[jepebe]

This PR adds a detail column justified to the right of the tree view. It provides a simple way of showing additional information that relates to the node. Example use cases could be amounts, temperature, sizes and more.

The following image is the result of the example provided in the documentation.

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[willmcgugan]

Interesting! Where does the detail column start? Is it different for each row or aligned with the maximum extend of the labels?

Any change of a vide of this in action?

[jepebe]

The detail column is calculated by finding the number of spaces required to fit the detail label 3 cells from the right edge. spaces = widget width - guide width - label width - detail width - 3

The magic minus 3 was by eye-balling the result (😄)
The minimum number of spaces is 1.

I can create a video showing the result, any recommendations to what tool to use for that?

[jepebe]

DetailTree480p.mov

Sorry about the quality, there was a 10 MB limit to the size of the file.

I see now that the handling of narrow views are a bit sub-optimal.

[jepebe]

I've made the right margin adapt to the visibility of the scroll bar.

[jepebe]

Let me know if there is a preferred strategy to handle narrow views.

During implementation I thought about different ways of solving this, this was the simplest one with minimal impact. However, I recognise that it adds API changes that could be difficult to move away from if there are plans to solve the same problem in a more flexible way in the future. Let me know how I may adapt the implementation to better accommodate such plans.

I can also add support for file sizes to the DirectoryTree in this PR (or another) if that is interesting.