Addition of category and rank as starter props

[imcovangent]

References

Issue #75

Code changes

Added two properties to the schema:

• category: category of the starter that becomes the header of the group in the launcher: defaults to Starters
• rank: control over the order of the starters in a certain category: defaults to Infinity

User-facing changes

Change is backwards compatible, but if category is given, then multiple headers are created in the launcher.

If rank is given, then the starters are sorted accordingly within the category.

Backwards-incompatible changes

N/A

Chores

• [x] linted
• [ ] tested
• [ ] checked on binder
• [ ] documented
• [ ] changelog entry

[imcovangent]

@bollwyvl This PR replaces https://github.com/deathbeds/jupyterlab-starters/pull/76

Do you have any pointers for adding tests? I'm not familiar with the .robot files that have been used.

All existing tests still run, but the new rank and category properties are now untested.

For the rest the PR is done.

[imcovangent]

FYI: I made a new PR since I messed up the branches. See: https://github.com/deathbeds/jupyterlab-starters/pull/76

That PR has processed the comments from that PR.

[bollwyvl]

This was manually merged into #86, and will be part of #87. Thanks!