Implement drag source on images

Using a 128x128 thumbnail.

src/widgets/image.rs

@@ -210,4 +210,22 @@ impl LpImage {
         let imp = self.imp();
         imp.texture.borrow().clone()
     }
+
+    pub fn content_provider(&self) -> anyhow::Result<gdk::ContentProvider> {
+        let imp = self.imp();
+        let mut contents = vec![];
+
+        if let Some(file) = imp.file.borrow().as_ref() {
+            let content = gdk::ContentProvider::for_value(&file.to_value());
+            contents.push(content);
+        }
+
+        if let Some(texture) = imp.texture.borrow().as_ref() {
+            let bytes = texture.save_to_png_bytes();
+            let content = gdk::ContentProvider::for_bytes("image/png", &bytes);
+            contents.push(content);
+        }
+
+        Ok(gdk::ContentProvider::new_union(contents.as_slice()))
+    }
 }