inspect: Implement pretty printing of GDateTime
inspect: Implement pretty printing of GDateTime

The new Zeitgeist work in the Telepathy backend makes use of nullable
GDateTimes, so this implements pretty printing of them in folks-inspect.

Everyone prefers reading well-formatted ISO 8601 date/times over reading
error messages saying the conversion wasn’t possible, right?

renatofilho/folks,renatofilho/folks2,pwithnall/folks,pwithnall/folks,renatofilho/folks,renatofilho/folks,GNOME/folks,pwithnall/folks,renatofilho/folks2,renatofilho/folks2,renatofilho/folks,renatofilho/folks2,GNOME/folks,GNOME/folks