Add UI element describing the state of the SDK connection

[emilk]

We have the re_smart_channel crate that tells us what we are getting data from (from a streaming Python SDK, a File, the network, …). We should have a UI element that shows what we are connected to (perhaps only on hover) and whether more data is still coming (e.g. is the file still loading, or the Python app still running?).

I think it would make sense to but this widget close to the playback controls (play, pause etc). It could be a single icon showing the type of thing we are connected to (what re_smart_channel::Source, file, http-stream, sdk, …) and some sort of modifier to show it is still active. When hovered more data is revealed (the name of the loaded file, for instance).

[abey79]

Mostly solved by #4443 isn't it?