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/package_list.cpp

@@ -73,7 +73,7 @@ void PackageList::showData(const String& pattern) {
   FOR_EACH(p, matching) {
     // open image
     PROFILER(_("load package image"));
-    InputStreamP stream = p->openIconFile();
+    auto stream = p->openIconFile();
     Image img;
     Bitmap bmp;
     if (stream && image_load_file(img, *stream)) {