Add @availability parameter to members in PXSourceList.h.
Change @availability references in PXSourceList{Delegate,DataSource}.h to refer to PXSourceList version.
Document more methods of PXSourceListDelegate.
Add note to the PXSourceListDelegate class documentation about `NSTableColumn` parameters not being a part of some of the delegate methods.
Document some of the new PXSourceListDelegate methods added in PXSourceList 2.0.