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/symbol/editor.hpp

@@ -30,7 +30,7 @@ class SymbolEditorBase : public IntrusivePtrVirtualBase {
   inline SymbolP getSymbol() { return control.getSymbol(); }
   
   /// Perform an action
-  void addAction(Action* action, bool allow_merge = true);
+  void addAction(unique_ptr<Action> action, bool allow_merge = true);
   
   void SetStatusText(const String& text);