bundle Adwaita symbolic icons in gresource

On macOS the Adwaita icon theme is not installed by default, so
symbolic icons (edit-copy, auth-fingerprint, network-wired,
dialog-warning, etc.) render as the "image-missing" placeholder.

Bundle the symbolic SVGs used by the GTK frontend into the embedded
gresource so the app is self-contained and doesn't depend on any
system-installed icon theme. The existing
`IconTheme::add_resource_path("/de/feschber/LanMouse/icons")` call
already tells GTK to search this prefix, so no code changes are
needed.

Icons are sourced from Adwaita and placed under the standard
`scalable/{actions,devices,places,status}/` hicolor layout.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lan-mouse-gtk/resources/icons/scalable/actions/process-stop-symbolic.svg

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg height="16px" viewBox="0 0 16 16" width="16px" xmlns="http://www.w3.org/2000/svg">
+    <path d="m 3 2 c -0.265625 0 -0.519531 0.105469 -0.707031 0.292969 c -0.390625 0.390625 -0.390625 1.023437 0 1.414062 l 4.292969 4.292969 l -4.292969 4.292969 c -0.390625 0.390625 -0.390625 1.023437 0 1.414062 s 1.023437 0.390625 1.414062 0 l 4.292969 -4.292969 l 4.292969 4.292969 c 0.390625 0.390625 1.023437 0.390625 1.414062 0 s 0.390625 -1.023437 0 -1.414062 l -4.292969 -4.292969 l 4.292969 -4.292969 c 0.390625 -0.390625 0.390625 -1.023437 0 -1.414062 c -0.1875 -0.1875 -0.441406 -0.292969 -0.707031 -0.292969 s -0.519531 0.105469 -0.707031 0.292969 l -4.292969 4.292969 l -4.292969 -4.292969 c -0.1875 -0.1875 -0.441406 -0.292969 -0.707031 -0.292969 z m 0 0" fill="#2e3436"/>
+</svg>