Implement hub callback priorities

[rosteen]

This PR allows users to set a priority for the hub callback when subscribing to a hub message, e.g.:

self.hub.subscribe(self, SubsetCreateMessage,
                           handler=lambda _: self._on_subset_created(),
                           priority=100)

I implemented this with a default priority of 10, where lower integer values are higher priority. I confirmed that this PR (and the code snippet above in Jdaviz) fixes the bug with the Layer not being populated before our data menu processes the SubsetCreateMessage.

@astrofrog @dhomeier There are two failing tests, I haven't figured out yet why those are now failing, so if you have thoughts on that or the implementation in general let me know.

[astrofrog]

Looks promising! Just a couple of comments and then if you could look into the tests that'd be great, but if you don't have time, just let me know and I'll see if I can investigate.

[dhomeier]

I only see the one failed test on Mock called twice; tried with @astrofrog's suggestion and removing the
priorities.append(priority), but there are still two calls (apparently with the correct message though).

[rosteen]

I only see the one failed test on Mock called twice; tried with @astrofrog's suggestion and removing the priorities.append(priority), but there are still two calls (apparently with the correct message though).

Oh good, I was seeing two locally, the second must have been due to something else in my local setup.

[rosteen]

Looks promising! Just a couple of comments and then if you could look into the tests that'd be great, but if you don't have time, just let me know and I'll see if I can investigate.

I fixed the failing test, the second loop to yield the handlers shouldn't have been inside the loop to populate the prioritized_handlers

[astrofrog]

Ideally it would be good to add at least one test for this - is that something you think you would be able to look into @rosteen or should @dhomeier or I look into it?

[rosteen]

Ideally it would be good to add at least one test for this - is that something you think you would be able to look into @rosteen or should @dhomeier or I look into it?

If @dhomeier has time today that would be awesome, otherwise I can try to figure something out tomorrow.

done?

[dhomeier]

Just some corrections to docstrings, otherwise looks good now, thanks!

[dhomeier]

Excellent, thanks!