Allow the message structure view to grow/shrink as much as sensible

* Ignore vertical size hint of HTML status bar to allow the message structure
  view to grow as much as possible.
* Set minimum height of message structure view to 10 pixels. This
  allows the message structure view to shrink to a very small height.
  Allowing it to shrink to zero height is not useful because then the user
  cannot resize it anymore.

Sponsored by: Intevation GmbH

BUG: 436044
FIXED-IN: 5.18.0