Create SlotFills

[bonny]

In order for add-ons to be able to add functionality we need to add support for SlotFills.

Docs:

• https://developer.wordpress.org/block-editor/reference-guides/components/slot-fill/
• https://developer.wordpress.org/block-editor/reference-guides/slotfills/

Tasks:

• [x] Always show events controlbar
• [x] Add SlotFills in toolbar
• [ ] Add SlotFills for each event