Palette: Remember category open/close state across reloads.

[dreamair]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

If you are working on some custom nodes and have them in a new category at the bottom of the list, it's nice to have the others closed even after a page reload.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
  They seem to fail, but not because of this change.
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage remained the same at 68.193% when pulling a118272 on dreamair:feature/remember-category-state into 30ea300 on node-red:dev.

[GogoVega]

Hi Bruno, thank you for your contribution.
@knolleary This PR is part of what was introduced in #4634. It can therefore be closed as superseded by #4634.