Grouped blocks

[musshush]

• [x] Support grouped blocks
• [x] Group blocks are always top level
• [x] Add group item
• [x] Remove group item
• [x] Re-order group item
• [x] Switch a group component to other supported group component
• [x] UI
• [x] Group and group item blocks always have outline
• [x] Expand and collapse group item block
• [x] Highlight selected block
• [x] Preview group block
• [x] Basic list component
• [x] Separate out config

[musshush]

@jonsharratt @charypar @sjltaylor @dwynne This is ready for a review. :dancers:

[musshush]

Gah, autocomplete! the last build comment should read 'dist', not 'diet'... :anger:

While waiting for a review, just fixed some build issues...