Improve Quickedit Context to make custom column callable optional

[mboynes]

There are two parts to this issue:

• Figure out how to make it work without adding a custom column
• Build a default callable, which can output data automatically

We add a custom column in order to populate the fields with the post's existing data. However, there are plenty of situations where someone wouldn't want a custom column -- there must be a workaround for this. It's possible to hide the column using Screen Options, so even if the solution is to hide it by default, that would probably be good enough.

The Quickedit context should also have a default callable which simply outputs the data. The context should be able to figure out, based on the field data, how best to display that data.