Patch preview: cancel notification

[Guillaume Emont <gemont igalia com>]

Here is what I have started for cancel notification. The base is there, and
proper cancellation notification should be implemented for browse.
I haven't tested it in any way, it's not finished code.

The reason I'm posting it, is to check I'm "on the right track", especially for
patch 3/3. The (asynchronous) code around that is not trivial, so, people who
understand it, please have a look at patch 3, and comment[1]. In the meantime,
I'll continue my work in the same fashion for the other operations.

Guij

[1] Yes Iago, I'm looking at you ;)