WIP: 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
For now, I would add
```
elif isinstance(col, (Time, SkyCoord)):
    is_mixin = True
```
With that, we do not have to touch `Quantity`, `Time`, and `SkyCoord` at all, which I think is better as we will still experiment with what is the best way to indicate a column can be used as a mixin. At the same time, by keeping the `else:` clause we can have easy experiments with other types of objects by jut setting `object._astropy_mixin_column = True`.