task.js: cooperative multitasking for long-running tasks

This is principally about task.js, which implements Long_Task, a class to run long-running tasks using a cooperative multitasking model. Other changes really just support this one.

counter_task.js is a very simple task generator that counts up to a limit. It's a testing example more than anything. Similarly task_test_ui.js. crawler.xul got a tab set.

NOTE: Because a defect, probably within IDEA, when I renamed crawler.js to crawler_ui.js (there were two files with the same name in different directories), it got committed to the repository as a file delete/add rather than a file rename. The hgreview extension proceeded to choke on this situation, aborting entirely rather than doing something sensible. Thus there are some very small initial changes missing from this changeset. This seemed better than not having a changeset at all.

[Felix Dahlke, 2013-06-04 14:11:09]

Says "Bad Content" for me, can't look at the code :(

Is this something we should review separately or shouldn't we just do it as part
of the big Crawler review?

[Eric, 2013-06-05 18:04:05]

On 2013/06/04 14:11:09, Felix H. Dahlke wrote:
> Is this something we should review separately or shouldn't we just do it as
part
> of the big Crawler review?

No need to do it separately. It's not even the most recent version of the code
(although it's reasonably close).

I'll close this review, given that it's been superseded.