Chat bubbles/heads

### Your use case

#### What would you like to do?
It would be nice to be able to chat with others via Element with the Bubbles chat API, just like Facebook Messenger.

#### Why would you like to do it?
To chat with others without constantly switching apps.

#### How would you like to achieve it?
I don't know, but there has already been an attempt to implement this in the regular Element.

Android provides the Bubbles API: https://developer.android.com/develop/ui/views/notifications/bubbles
Element-android feature request: https://github.com/vector-im/element-android/issues/1124
Signal's code implementing such a feature: https://github.com/signalapp/Signal-Android/commit/e1bf23251f49d4cd311615fe1c3cbeee685e6835

### Have you considered any alternatives?

No

### Additional context

No

### Are you willing to provide a PR?

No