feat(gui,core): modern Photos layout

Lay the timeline out in justified rows. Each photo keeps its own shape,
clamped between 1:2 and 5:2, and every row but a section's last spans
the width to the pixel. "Square Tiles" in the page menu restores the
even grid, and the Year zoom stays square. The zoom level and the
tile shape persist in a new `photos_view` config section.

Move Timeline/Albums/Places into the header as an icon-and-label
switch that shows only icons in a narrow window. The filters take a
single row whose kind tabs scroll sideways.

Selection mode follows GNOME apps:
- the header shows "N selected" and Cancel;
- an ActionBar along the bottom offers Favorite (or unfavorite), Add
  to Album, Remove from Album, Save Copies and Move to Trash;
- each section heading gets a check that selects the whole section.
One function, sync_header, decides the header for the page, an open
album or place, and selection mode.

The heading of the section at the top of the grid floats over the
grid. Favorites get a star badge, and video placeholders a video
glyph. New bundled icons: pdfs-photos, pdfs-album and pdfs-slideshow.

Upload takes several photos and videos at once, and files dropped on
the timeline upload too. The media type comes from the MIME database,
except that a recording-sized .ts counts as video.

The viewer's details panel moves into an OverlaySplitView that lies
over the photo below 800sp.

docs/CHANGELOG.md

@@ -37,6 +37,30 @@ Existing folders remain device backups; older builds cannot open the upgraded da
   Otherwise the daemon takes a frame with ffmpeg from the start of the file, plus an MP4's
   index, and never downloads more than about 16 MiB of a video for it, however large the
   video is. **Build Thumbnails** covers videos too (B283).
+- **A new layout for Photos.**
+  - The grid shows every photo at its own shape, in rows that fill the width (portraits
+    are no longer cropped to squares). **Square Tiles** in the page menu brings the even
+    grid back, and the Year zoom stays square.
+  - Timeline, Albums and Places switch in the header. The filters take one row, which
+    scrolls sideways in a narrow window.
+  - The section you are in floats over the top of the grid once its heading has scrolled
+    away. Headings are larger, and a check beside each one selects the whole day, month or
+    year.
+  - Selection mode works as in other GNOME apps: the header shows the count and
+    **Cancel**, and a bar along the bottom has these actions:
+    - **Add to Favorites**, or remove them when every selected photo is already one;
+    - **Add to Album**;
+    - **Remove from Album**;
+    - **Save Copies**, into a folder you pick, never overwriting a file;
+    - **Move to Trash**.
+  - Favorites carry a star on their tile, and a video waiting for its thumbnail shows a
+    video glyph.
+  - New icons for Photos, albums and the slideshow.
+  - **Upload** takes several photos and videos at once, and you can drop files onto the
+    timeline to upload them.
+  - The zoom level and the tile shape are remembered between runs.
+  - In the photo viewer, the details panel lies over the photo when the window is
+    narrow instead of squeezing it.
 
 ### Fixed