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/render/value/image.cpp

@@ -32,7 +32,7 @@ void ImageValueViewer::draw(RotatedDC& dc) {
     // load from file
     if (!value().filename.empty()) {
       try {
-        InputStreamP image_file = getLocalPackage().openIn(value().filename);
+        auto image_file = getLocalPackage().openIn(value().filename);
         if (image_load_file(image, *image_file)) {
           image.Rescale(w, h);
         }

src/render/value/package_choice.cpp

@@ -37,7 +37,7 @@ void PackageChoiceValueViewer::initItems() {
     i.package_name = p->relativeFilename();
     i.name = capitalize_sentence(p->short_name);
     Image image;
-    InputStreamP stream = p->openIconFile();
+    auto stream = p->openIconFile();
     if (stream && image_load_file(image, *stream)) {
       i.image = Bitmap(resample(image, 16,16));
     }