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
Seeing one of your tests checking that the `unit` on any `mixin` column can be set, I wondered whether it should actually be among the standard attributes. I think I would for now leave it as one that should be on the object itself. I *think* all this entails is removing it here and checking for `getattr(col, 'unit`)` but am not quite sure.