feat(gui): Ctrl+scroll changes the icon size in My files

Scrolling with Ctrl held over the icon grid steps the icon size, as it
does in Photos. Touchpad deltas add up to a whole notch before each
step, so a small swipe does not jump through every size.

docs/CHANGELOG.md

@@ -62,8 +62,8 @@ Existing folders remain device backups; older builds cannot open the upgraded da
   - In the photo viewer, the details panel lies over the photo when the window is
     narrow instead of squeezing it.
 - **Larger icons in My files.** The icon size slider goes up to 256 px, big enough to
-  tell photos apart by their thumbnail. Ctrl+plus, Ctrl+minus and Ctrl+0 change it from
-  the keyboard, and the size is remembered between runs.
+  tell photos apart by their thumbnail. Ctrl+scroll, Ctrl+plus, Ctrl+minus and Ctrl+0
+  change it too, and the size is remembered between runs.
 
 ### Fixed