internal/ui: notify when a new incoming file arrives

[DeedleFake]

This is a bit tricky for two reasons:

• It needs to properly detect new incoming files. Two files arriving with the same name and size could cause this to be a bit messy. Or maybe that's not a problem.
• It probably shouldn't just spam a ton of notifications when Trayscale first starts if there are a bunch of files pending already.

[Jacalz]

This would be a very useful feature to have :)