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
p.s. Of course, this would need local imports of `Time` and `SkyCoord` -- I think that is fine since by this time `is_mixin` will generally have been set already. So, maybe:
```
else:
    is_mixin = getattr(col, '_astropy_mixin_column', False)
    if not is_mixin:
        from ..time import Time
        from ..coordinates import SkyCoord
        is_mixin = isinstance(col, (Time, SkyCoord))
```