Give Effector its own icon

An E split into its colour channels, with two slices slipped sideways,
replaces the generic applications-graphics icon in the desktop entry and
About Effector, with a one-colour symbolic version for panels. Both are
installed into the hicolor theme; a build run straight from the build tree
falls back to the copy in the sources. The version moves to 0.5.3.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/gui/app.cpp

@@ -13,6 +13,14 @@ int runGui(const Options& initial, int argc, char** argv) {
 
     auto app = Gtk::Application::create("org.effector.Effector",
                                         Gio::Application::Flags::NON_UNIQUE);
+    app->signal_startup().connect([] {
+        // An installed Effector finds its icon in the hicolor theme; one run
+        // straight from the build tree falls back to the copy in the sources.
+        const char* icon = "org.effector.Effector";
+        auto theme = Gtk::IconTheme::get_for_display(Gdk::Display::get_default());
+        if (!theme->has_icon(icon)) theme->add_search_path(EFFECTOR_SOURCE_ICON_DIR);
+        Gtk::Window::set_default_icon_name(icon);
+    });
     return app->make_window_and_run<EffectorWindow>(0, nullptr, initial);
 }