feat(gui,core): larger icon sizes in My files

The icon grid's size slider stops at 256 px instead of 144 px, which is
big enough to tell photos apart by their thumbnail. Thumbnails are
512 px, so they stay sharp at that size on a 2x screen. Names may wrap
wider on the larger tiles.

Ctrl+plus, Ctrl+minus and Ctrl+0 step and reset the size while the grid
is shown, as they do in Photos. The size is saved in the config as
files_view.icon_size once the slider rests; configs without it keep the
72 px default.

docs/CHANGELOG.md

@@ -61,6 +61,9 @@ Existing folders remain device backups; older builds cannot open the upgraded da
   - 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.
+- **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.
 
 ### Fixed