Add support for opacity in ShellClient

shell_client.cpp

@@ -266,12 +266,19 @@ NET::WindowType ShellClient::windowType(bool direct, int supported_types) const
 
 double ShellClient::opacity() const
 {
-    return 1.0;
+    return m_opacity;
 }
 
 void ShellClient::setOpacity(double opacity)
 {
-    Q_UNUSED(opacity)
+    const qreal newOpacity = qBound(0.0, opacity, 1.0);
+    if (newOpacity == m_opacity) {
+        return;
+    }
+    const qreal oldOpacity = m_opacity;
+    m_opacity = newOpacity;
+    addRepaintFull();
+    emit opacityChanged(this, oldOpacity);
 }
 
 void ShellClient::addDamage(const QRegion &damage)

shell_client.h

@@ -170,6 +170,7 @@ private:
     bool m_fullScreen = false;
     bool m_transient = false;
     bool m_internal;
+    qreal m_opacity = 1.0;
 };
 
 }