PPTX footer export & import interoperability

1. What were the initial problems
   1. Footers imported as textboxes (not as Impress understands them)
   2. Footers don't get exported
2. The fix
   1. If possible import footers as properties (as Impress understands them) if not fallback to the previous implementation
   2. Export footers
      • PPTX placeholder inheritance
      • lstStyles (on master placeholders)
      • Placeholder indexes (for slide placeholders)
   3. Better the datetime conversion
3. What's left?
   1. Right now only for slides, not for notes/handouts
   2. Footer placeholders use the lstStyles but it could be expanded to all of the placeholders, (maybe for better interoperability)