The space panel needs some kind of 'unread' indicator

#### Is your feature request related to a problem? Please describe.

On Element Web, my workflow is this: I open up the space panel, look for spaces that have an 'unread' dot on them to find out where there has been activity, and then visit each of them to catch up with messages. However, on Android there is no such indication of which spaces are unread, which means I have to guess or know ahead of time where there has been activity in order to get to it. This is not a huge deal now since you can still easily see which rooms are unread from the "All rooms" view, but once https://github.com/vector-im/element-android/issues/3754 is implemented they will become much harder to find.

#### Describe the solution you'd like.

There should be some kind of indicator displayed on spaces when they have unread messages, much like the dot that appears on Web: