ui: Add a header row API for plugins

Plugins can add sidebar entries and, per trace, status bar items, but no
full-width controls, e.g. those of a live stream, which must also show
while its trace opens. Add app.header, and trace.header whose items go
away with the trace. Items render as full-width rows above the topbar
and the sidebar, in registration order.

Without items nothing renders, so the layout doesn't change. The sidebar
button now aligns with the top of the sidebar's header instead of the
window's, so a header row moves it down with the sidebar. Hence, with
the hiring banner (off by default), it now sits below the banner.