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
The zero-order solution for implementing your idea would basically involve copying the entire `_init_from_cols` into `QTable`, and then deleting a few lines from the `Table` version.  I don't see how to nicely call a super method.  Then you would be left with two chunks of code that need to be kept synced.  Of course one could refactor the method into smaller bits that are common to both, but the whole initialization chain is already complicated enough.

Did you have a nicer solution in mind?