Consider rendering menus within editor DOM

[YousefED]

We currently append them to body. We should consider to position them within the dom of the editor. This could fix some manual-position issues like:

#167 (review)

[YousefED]

Possible related: #189

[matthewlipski]

Some findings from looking into this issue:

• TipTap extensions get initialized before the editor itself, so the .ProseMirror element we'd want to attach a menu to doesn't exist when we need to create the factory for it. Could just attach to a parent instead, not sure what the best solution is atm.
• In React, the editor is rendered from #root, which means that for any children, we can't really attach another root to render the menus. We would have to pass the menus down to BlockNoteView and have that sort out the rendering. This would require quite a lot of changes because showing/updating/hiding the menus is currently the responsibility of the corresponding PM plugins, but now BlockNoteView would be responsible for it.