Fixed: incorrect table column highlighting and row selection after column sort.

Previously in CPTableView, after clicking a sortable column, data views of the sorted column ended up in a selected state and a row was selected instead of the sorted column. As a result, the text of the rows would turn white as if they were selected, but there was no highlight, so the text was basically invisible.

This commit fixes that by reversing the order of operations when clicking a column. First the sort is done, then the selection.

Fixes #1337, #1688, #1779.
Fixed: improved handling of custom styled table text cells.

Previously more work was done than necessary to initialize custom table text cells.

Now the theme state is set before initializing table cells, which allows fonts and colors to be set correctly. The selected colors still have to be set explicitly.

BREAKING CHANGE:

Because this operates within nib2cib, all xibs should be re-converted.
Fixed: default class theme value could be CPNull.

The class method +themeAttributes returns a dictionary, so CPNull has to be used instead of nil. Previously, if the class theme attribute was used, there was no attempt to convert CPNull to nil, which could have unintended consequences.