Handle double click on Decoration title bar

Passed to the proper event handler in Workspace.

client.cpp

@@ -517,6 +517,12 @@ void Client::createDecoration(const QRect& oldgeom)
     if (m_decoration) {
         m_decoration->update();
         connect(m_decoration, &KDecoration2::Decoration::shadowChanged, this, &Toplevel::getShadow);
+        connect(m_decoration, &KDecoration2::Decoration::titleBarDoubleClicked, this,
+            [this]() {
+                Workspace::self()->performWindowOperation(this, options->operationTitlebarDblClick());
+                dontMoveResize();
+            }
+        );
     }
 
     move(calculateGravitation(false));