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/data/add_cards_script.cpp

@@ -47,6 +47,6 @@ void AddCardsScript::perform(Set& set) {
   // Add to set
   if (!cards.empty()) {
     // TODO: change the name of the action somehow
-    set.actions.addAction(new AddCardAction(ADD, set, cards));
+    set.actions.addAction(make_unique<AddCardAction>(ADD, set, cards));
   }
 }