Fixed: highlighted table cells with CPTableViewSelectionHighlightStyleNone had white text.

Previously, CPTableView was unconditionally setting the selected theme state for selected data views. When the selection highlight style is CPTableViewSelectionHighlightStyleNone, there is no row selection highlighting done. Since the default theme specifies white text for selected data views, we ended up with white text on a non-highlighted background, which was invisible on non-alternate rows and almost invisible on alternate rows.

This commit unsets the selected theme state flag for any redrawn data views when the selection highlight style is CPTableViewSelectionHighlightStyleNone, thus the text color does change at all when selected.

Fixes #1847
New: CPTableView public -enumerateAvailableViewsUsingBlock: and private _enumerateViewsInColumns:rows:usingBlock:

These methods allow to enumerate visible data views or data views in specified columns and rows.
-enumerateAvailableViewsUsingBlock: is the counterpart of cocoa's -enumerateAvailableRowViewsUsingBlock: except that it enumerates data views instead of CPTableRowView and the block has an additional column parameter.

Applied the second method where it is relevant, making the code more compact and readable.