Fix signals for abstract models

Includes a fix for undispatched signals, as well as signals being
handled twice.

A regression of #126
Related: #185
Add test for abstract model signals
Add test for @seanbell's utility

Currently failing
Fix get_nonabstract_descendants

Previously, function was only returning the first non-abstract
descendant. (It should also return concrete descendants of concrete
descendants, which it now does.)

Also, converted to generator.
Clarify ImageFieldSourceGroup docstrings
Merge branch 'abstract-model-fixes' into develop
Bump version to 3.0a3