glib: Provide accessor functions for PopplerAttachment and seal the struct in future
PopplerAttachment has public struct members that contain the attachment's data fields (name, desc, dates etc).
The modern GObject idiom is to have no publicly accessible fields in object structs, and instead provide all data via accessor functions and GObject properties.
There is also a further problem here in that PopplerAttachment contains 2 timestamps (ctime, mtime) as GTime which is 32-bit. The accessor functions that replace the struct access should return a 64-bit time value, or, (better IMHO), GDate.
Once the accessor functions are provided, we should mark the struct fields as deprecated.