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
But `nomask` doesn't allow assignment of `False` either.  The intent with `FalseArray` was to make a attribute that looks like the normal masked array `mask` in most every way (e.g. has length and shape, etc) except that you can't assign True to any element (but assigning False is fine).  If there is an existing class or you think there is a nicer way to finesse the issue of masking in mixin cols then I'm definitely all ears.