Cancel a scan in progress

[btsouth]

There was no way to stop a long scan, say a whole disk, short of quitting or opening another folder. Now Escape, or a Cancel button on the scanning panel, stops it.

A cancelled first scan leaves the panel saying it stopped, with the counts so far and a Scan again button (r works too). The partial tree is never shown, since it would read like a finished result. Escape during a widening scan (g) stops it and keeps the tree already on screen. An active filter still takes the first Escape. The key bar, the help overlay and the README mention it.

with_tree, the test constructor, now cancels the walk that new starts, as its doc comment already says it does. Without that, Escape in the existing key tests cancelled the leftover walk instead of going up a level.

Tests: window tests for a cancelled first scan and a cancelled widening. I also tried it by hand on a volume with about 1.1M files. cargo xtask lint and cargo xtask test pass.