Open a new one with SUPER, go to the old one with SUPER+ALT

Making the plain application keys focus an existing window was the
wrong call. It reads well in a demo and it is what macOS does, but it
made "give me another terminal" the awkward case -- and on a tiling
desktop a second terminal beside the first is the normal way to work,
not an edge case. Reaching for the launcher to open a second file
manager is not an improvement on anything.

So the plain keys do what they always did, and SUPER+ALT is the new
capability rather than a tax on the old one: go to the terminal,
editor, browser, files, calculator or mail you already have, wherever
it is, and start one only if there is none.

ALT rather than SHIFT because SUPER+SHIFT is already the
window-manipulation space -- Files, Neovim and Settings would have
collided with Focus session, Taller and Shorter, and breaking two keys
out of the eight-key resize set to make room is the worse trade.

Also fixes a real trap found while using it. The Alt-Tab overlay
commits on SUPER release, which is a compositor bind running an IPC
call; if that call ever fails to land, the overlay stayed up with no
keyboard focus, no Escape handler and nothing clickable, so the only
way out was an IPC call typed into a terminal it was covering. Clicking
outside now dismisses it, clicking a row switches to that window --
which is the obvious thing to try and did nothing -- and an abandoned
switch closes itself after ten seconds. Keyboard focus still stays with
the compositor, because taking it mid-switch is what would break
stepping.

README.md

@@ -114,7 +114,7 @@ docs/           Settings reference, and the design specs behind the work
 
 ## Tests
 
-147 of them, under `tests/`. Run the lot, or a subset by pattern:
+148 of them, under `tests/`. Run the lot, or a subset by pattern:
 
 ```sh
 panama test           # everything