Add a custom layer maker registry

[keflavich]

cc @astrofrog, @catherinezucker - this is infrastructure required for the region plugin.

WIP - do not merge!

[astrofrog]

@keflavich @catherinezucker - I'm going to be cutting a 0.14 release early next week. Are you interested in finishing this PR up by then, or will we just keep this for the next release?

[keflavich]

I don't have time to work on it; what more is needed though? Do we need some sort of tests for this?

[astrofrog]

@keflavich - I'm happy to finish it up and add a few tests. Could you rebase first though, as I think it will fix the CI?

[codecov]

Codecov Report

Merging #1850 into master will increase coverage by 0.04%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #1850      +/-   ##
==========================================
+ Coverage   85.51%   85.56%   +0.04%     
==========================================
  Files         235      235              
  Lines       20899    20949      +50     
==========================================
+ Hits        17871    17924      +53     
+ Misses       3028     3025       -3

Impacted Files	Coverage Δ
glue/core/application_base.py	83.87% <ø> (-0.09%)	⬇️
glue/app/qt/application.py	78% <100%> (+0.1%)	⬆️
glue/core/layer_artist.py	78.88% <60%> (+0.89%)	⬆️
glue/config.py	81.79% <84.61%> (+0.1%)	⬆️
glue/viewers/common/viewer.py	92.2% <93.33%> (+2.16%)	⬆️
glue/core/subset.py	88.03% <95.45%> (+0.29%)	⬆️

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[astrofrog]

@keflavich @catherinezucker - I've finally had a chance to finish this up and add tests and docs. Note that I've changed how the custom layer artist stuff works a little to make it more general. Basically you need to write a function that takes a viewer and data or subset object, and returns either a layer artist or None. So for the regions plugin, you'll need to adjust this. See the docs in this PR.