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/format/apprentice.cpp

@@ -301,7 +301,7 @@ void ApprDistroDatabase::removeSet(const String& code) {
   // TODO ?
 }
 
-void ApprDistroDatabase::doRead(InputStream& in) {
+void ApprDistroDatabase::doRead(wxInputStream& in) {
   wxTextInputStream tin(in);
   ApprDistro* last = 0;
   while (!in.Eof()) {

src/data/format/clipboard.cpp

@@ -23,16 +23,16 @@
 /// Serialize an object to a string, clipboard_package will be set to the given package.
 template <typename T>
 String serialize_for_clipboard(Package& package, T& object) {
-  shared_ptr<wxStringOutputStream> stream( new wxStringOutputStream );
+  wxStringOutputStream stream;
   Writer writer(stream, file_version_clipboard);
   WITH_DYNAMIC_ARG(clipboard_package, &package);
     writer.handle(object);
-  return stream->GetString();
+  return stream.GetString();
 }
 
 template <typename T>
 void deserialize_from_clipboard(T& object, Package& package, const String& data) {
-  shared_ptr<wxStringInputStream> stream( new wxStringInputStream(data) );
+  wxStringInputStream stream = {data};
   Reader reader(stream, nullptr, _("clipboard"));
   WITH_DYNAMIC_ARG(clipboard_package, &package);
     reader.handle_greedy(object);