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/image.cpp

@@ -65,7 +65,7 @@ bool ImageValueEditor::canPaste() const {
 
 bool ImageValueEditor::doCopy() {
   // load image
-  InputStreamP image_file = getLocalPackage().openIn(value().filename);
+  auto image_file = getLocalPackage().openIn(value().filename);
   Image image;
   if (!image_load_file(image, *image_file)) return false;
   // set data