epubveri for calibre 0.4.1

Both of these are the panel's background setting, which 0.4.0 shipped this
morning and nobody had looked at until thiago.eec ran it on Windows. Released
the same day for that reason: the fix cannot be verified where it was written,
and there is somebody testing right now.

• A chosen background now reaches the tree, not just the dock.
  thiago.eec, post 26: "when you choose 'Always dark' or 'Always light' for
  the background, only the dock gets repainted, the tree widget remains the
  same color as in calibre theme." The panel told the tree in the one way
  that does not arrive everywhere — a palette, which a platform style is free
  to ignore for an item view, and Windows does. It is told twice now, the
  second time with a style sheet naming the same two colours, which Qt draws
  itself. Not reproducible on macOS under either style, so this is
  reasoned from where the two platforms differ rather than from a failure
  seen here; the per-row severity colours and the selection colour were
  measured to be untouched by it.

• "Follow calibre" takes a background off again. Choosing Always dark
  and then going back to Follow calibre left the panel dark until calibre
  was restarted — and left the dark severity colours with it, since the row
  colours are decided by reading the panel's own background back. One panel
  is built per editor session and reused, so declining to paint a background
  was not the same as removing one. Nobody reported this; it was found while
  looking into the report above.