Rename to wl-pick

wlgrid described the shape of the thing rather than what it is for, and
the shape is the least interesting part now that it doesn't focus
anything: it shows you what is running and reports which one you pointed
at. wl-pick says that.

The wl- prefix rather than wlr- is deliberate. The capture path is
entirely upstream ext- protocols (ext-image-copy-capture,
ext-image-capture-source, ext-foreign-toplevel-list); the only
wlroots-flavoured piece is layer-shell, which KDE, niri and labwc
implement too. What actually keeps this from running on other
compositors is the sway IPC dependency, not wlroots, so wlr- would
promise a portability that isn't there. Hyphenated because the ecosystem
hyphenates when the suffix is a real word — wl-clipboard, wl-mirror,
wl-screenrec — and reserves the smushed form for coinages like wlsunset.

Also renames the layer-shell namespace and the memfd labels, which show
up in compositor debugging.

src/sway.rs

@@ -2,7 +2,7 @@
 //! Wayland side only supplies pixels. The join between the two is
 //! `foreign_toplevel_identifier`, which sway reports per view.
 //!
-//! Acting on the choice is deliberately not here: wlgrid reports what was picked
+//! Acting on the choice is deliberately not here: wl-pick reports what was picked
 //! and the caller decides what that means.
 
 use swayipc::{Connection, Node, NodeType};