Introduce warning topbar menu, allow following crates to see warnings for

[Nemo157]

Based on top of #2470, only the last commit is relevant currently

I still need to finish up work on grabbing data for the actual warnings, but I wanted some early feedback on the approach I'm taking to the UI, given this is the most client-side bit of UI we've got, @GuillaumeGomez.

Preview of what it looks like (with my wip warnings rendering too):

[syphar]

for progress this would need to be rebased