feat(gui): drag photos out of the timeline, upload progress bar

- A drag that starts on a selected tile carries the selection out as a
  text/uri-list. PhotoDrag, a gdk::ContentProvider, downloads the photos
  only when the receiver reads the list, and hands over hard links (or
  copies) under their Drive names in the user cache; copies older than
  a day are pruned. Drag-select steps aside for such a drag, and the
  timeline's own drop target ignores drags from inside the app.
- Uploading several photos shows "Uploading 3 of 12 items…" with a
  progress bar under the Photos header, replacing the start toast.
  Photo uploads report no byte progress from the daemon, so the bar
  counts files; per-tile rings would need that first.

docs/CHANGELOG.md

@@ -88,6 +88,13 @@ Existing folders stay device backups. Older builds cannot open the upgraded data
     slideshow.
   - The line under the date names the town the photo was taken in, when it is known.
 
+- **Drag photos out of Photos.** Select photos, then drag one of them into a file manager, a chat
+  or any app that takes files. The photos download when dropped and arrive under their own
+  names. A drag that starts on a photo that is not selected still selects, as before.
+
+- **Upload progress in Photos.** Uploading several photos at once shows "Uploading 3 of 12
+  items…" with a progress bar above the timeline, instead of a single toast at the start.
+
 ### Changed
 
 - **`proton-sdk` and `proton-drive-rs` 0.11.0.** Timestamps are converted where they enter