Add playlist export action to Playlist Dock save action.

Let's summarize what is wrong with the current UI design (from my standpoint):
1. the playlist dock is the only dock that has it's own menu entry
2. inconsistency between the toolbar and the menu. You need to know where
  to find the option
3. toolbar button with multiple options is evil. You need to do a long
  press and the only indication that a user can do this is a tiny arrow.
4. toolbar is hidden as the playlist tab has in principle two. Hidden in
  this respect means that it's easy to overlook since people normally read
  from the top and the playlist dock has already an option bar at the top.
  So why look at the bottom?

In principle this patch improves on point 2.

FEATURE: 105824
FIXED-IN: 2.8