Clean up pointer use:

* Use unique_ptr for Actions instead of manual memory management
 * Use unique_ptr in KeywordDatabase
 * Use unique_ptr instead of shared_ptr for file formats
 * Don't pass shared_ptr to Reader/Writer, use references instead
Also
 * Switch to C++17 so we can use map::try_emplace

src/gui/set/cards_panel.cpp

@@ -325,7 +325,7 @@ void CardsPanel::onCommand(int id) {
       if (card_list->canSelectNext()) card_list->selectNext();
       break;
     case ID_CARD_ADD:
-      set->actions.addAction(new AddCardAction(*set));
+      set->actions.addAction(make_unique<AddCardAction>(*set));
       break;
     case ID_CARD_REMOVE:
       card_list->doDelete();