Support Table mixin columns

This is essentially the same as #1833, #1835, #1839 but will be done in the framework of #2790 as a follow-on once that is merged.

The starting point here will be fed00dc5f6d6d9529ed8f073c95346eb176fdfa6, which *removed* the proof-of-concept changes showing that mixin columns would be straightforward in the new column-based Table structure.   In this issue (and eventual PR) we can define the right way to support mixin columns like Quantity, Time, and SkyCoord.  The original proof-of-concept used a some simple class / instance variables, but @mhvk has proposed using an abstract base class.

Closes #1833
Closes #1835
Closes #1839
I agree there is a problem and I struggled a bit with `unit` and `dtype`.  Once again I think this will all go away with the `ColumnAttributes` class (that was a good idea!).  The key is that the mixins can then subclass and customize the dispatch method for the different attributes.

In the meantime I'm inclined to let this be in the current implementation.  Who knows, maybe `ColumnAttributes` will appear in the next day or two... but we want something merge-able by tomorrow.