Element X - Custom Widget Support

[brcourt]

I'd like custom widgets to be supported on mobile clients.

I'm working on an Immich (photo hosting app) bridge/widget that will sync photo albums with rooms and room membership syncs with album share permissions. Images added to the room get uploaded to the album and photos uploaded to the album post a thumbnail in the room (bidirectional). The widget on Element Desktop shows a photo-wall and I can chat with my family members about the images they upload, and I get push notifications when someone uploads to an album shared with me. Using Matrix chat makes it much more convenient to comment on photos which in turn, helps make photos more searchable in Immich.

I would very much like to the ability to configure rooms with custom widgets on Element X. On a mobile client, it would make sense that widgets render full screen, replacing the chat window. It also makes sense that widgets enabled in a room would get an icon in the top bar (next to the camera or phone icon) that opens the widget.