Files · Explorer: a folder as a list, tiles or columns, with one path and one order

FileExplorer shows the folder at the end of a path three ways: the
directory listing, a grid of tiles, or a column per folder along the
path with the next folder lit. Its name, its count and a switch of views
sit above one path bar that climbs. A folder opens in place; a file goes
to the host, which lists each folder it goes to. A pick in an earlier
column closes the columns past it, and a new deepest column scrolls into
view. The directory listing takes its pick from its owner and reports
it, so the pick carries across views; its path bar and its first order
are shared. The gallery walks a small file system and shows the tree,
the context menu, renaming in place and a drop zone in use.

src/primitives/icon.rs

@@ -109,6 +109,7 @@ icons! {
     CloudUpload => "cloud-upload",
     Code => "code",
     Columns2 => "columns-2",
+    Columns3 => "columns-3",
     Command => "command",
     Compass => "compass",
     Contrast => "contrast",