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/value/editor.cpp

@@ -12,9 +12,9 @@
 
 // ----------------------------------------------------------------------------- : ValueEditor
 
-void ValueEditor::addAction(ValueAction* a) {
+void ValueEditor::addAction(unique_ptr<ValueAction> a) {
   if (a) {
     a->isOnCard(editor().getCard().get());
-    editor().addAction(a);
+    editor().addAction(move(a));
   }
 }