Separate model and view code in the computed object and inspector tree

[naschmitz]

Before, get_info returned a widget representing the computed object. Now, get_info calls build_computed_object_tree to build an intermediate representation of the computed object. The intermediate representation is converted to a widget.

The advantage of this intermediate representation is:

• the majority of the tree traversal logic can be tested by comparing the resulting tree as JSON, and
• the tree can be reconstructed with different views (e.g. ipytree, custom inspector widget, etc.).

[sufyanAbbasi]

This is a sweet refactor! I think the code really improved in readability. I have a few more suggestions, otherwise, LGTM!

[giswqs]

@naschmitz I suggest using the Squash and merge for merging future pull requests. That can reduce the number of commits in the repo commit history and making it easier to include the pull request number in the release notes.