window: Support dropping multiple files

This allows to scroll through all dropped files.
No further images from directories are loaded in
this mode.

Closes #35

src/application.rs

@@ -73,7 +73,7 @@ mod imp {
             let application = self.obj();
             for file in files {
                 let win = LpWindow::new(&*application);
-                win.set_image_from_file(file);
+                win.image_view().set_image_from_file(file.clone());
             }
         }
     }