[wayland] Provide icon from ShellSurface's windowClass

We can use the window class to locate a desktop file and from this
we can load a themed icon.

shell_client.cpp

@@ -33,6 +33,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <KWayland/Server/plasmashell_interface.h>
 #include <KWayland/Server/qtsurfaceextension_interface.h>
 
+#include <KDesktopFile>
+
 #include <QWindow>
 
 using namespace KWayland::Server;
@@ -83,6 +85,8 @@ ShellClient::ShellClient(ShellSurfaceInterface *surface)
             maximize(maximized ? MaximizeFull : MaximizeRestore);
         }
     );
+    connect(surface, &ShellSurfaceInterface::windowClassChanged, this, &ShellClient::updateIcon);
+    updateIcon();
 }
 
 ShellClient::~ShellClient() = default;
@@ -627,4 +631,13 @@ bool ShellClient::hasStrut() const
     return m_plasmaShellSurface->panelBehavior() != PlasmaShellSurfaceInterface::PanelBehavior::WindowsGoBelow;
 }
 
+void ShellClient::updateIcon()
+{
+    if (m_shellSurface->windowClass().isEmpty()) {
+        setIcon(QIcon());
+    }
+    KDesktopFile df(QStringLiteral("%0.desktop").arg(QString::fromUtf8(m_shellSurface->windowClass())));
+    setIcon(QIcon::fromTheme(df.readIcon()));
+}
+
 }

shell_client.h

@@ -125,6 +125,7 @@ private:
     void createWindowId();
     void findInternalWindow();
     void updateInternalWindowGeometry();
+    void updateIcon();
     static void deleteClient(ShellClient *c);
 
     KWayland::Server::ShellSurfaceInterface *m_shellSurface;