Visual Page Builder improvements and fixes

A meta issue to keep track of actionable feedback from the recent released Visual Page Builder

## v0.1 (by priority)

- [x] BeaconCMS/beacon_live_admin#155
- [x] BeaconCMS/beacon_live_admin#154
- [x] BeaconCMS/beacon#506
- [x] BeaconCMS/beacon_live_admin#131
- [x] BeaconCMS/beacon_live_admin#152
- [x] BeaconCMS/beacon_live_admin#151
- [x] #176
- [x] #205
- [x] #203

### Nice to have
- [x] BeaconCMS/beacon_live_admin#132
- [x] #193

## Backlog

- [ ] BeaconCMS/beacon_live_admin#133
- [ ] BeaconCMS/beacon_live_admin#134
- [ ] BeaconCMS/beacon_live_admin#135
- [ ] BeaconCMS/beacon_live_admin#136
- [ ] BeaconCMS/beacon_live_admin#137
- [ ] BeaconCMS/beacon_live_admin#138
- [ ] Support markdown pages - duplicate of https://github.com/BeaconCMS/beacon_live_admin/issues/95
- [ ] Auto-complete Tailwind utility classes
- [x] #196

## Done

- [x] Javascript error clicking on the edit button for a children node on the list of children on the right sidebar. BeaconCMS/beacon_live_admin#113
- [x] Edit content in place (`contenteditable`?) (#121)
- [x] Be able to preview where the component being dragged is going to land. (#118)
- [x] Backlist "self_close" as a property that can be edited in the sidebar (#117)
- [x] Prevent default behavior of anchors in the layout (#116)
- [x] Change icon to move up and down the tree. The current one looks like an "Undo" button. (#120)

Review the unchecked backlog entries and their linked BeaconCMS/beacon_live_admin issues, especially #133 through #138 and the markdown-pages duplicate. Use those issue descriptions to identify the remaining scope; this meta issue is complete only when the outstanding actionable items are resolved or explicitly closed.

Written by the indexing model from the issue text.