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/control/card_editor.cpp

@@ -58,8 +58,8 @@ bool DataEditor::viewerIsCurrent(const ValueViewer* viewer) const {
 }
 
 
-void DataEditor::addAction(Action* action) {
-  set->actions.addAction(action);
+void DataEditor::addAction(unique_ptr<Action> action) {
+  set->actions.addAction(move(action));
 }
 
 // ----------------------------------------------------------------------------- : Selection