[Evolution] Image loading threads don't cancel

[Matt Needles <matt needleswebs com>]

When an HTML message is loading images, its thread will not cancel when
you change to another message. This is unacceptable behaviour. The
thread that loads parts of a message should cancel immediately when the
user changes to a different message. If it does not, the whole
application hangs, sometimes for a very long time. Even clicking the
cancel button on the thread does not stop it.