[styledock] open via double click action legend setting (#3220)

src/app/qgisapp.cpp

@@ -463,6 +463,9 @@ void QgisApp::layerTreeViewDoubleClicked( const QModelIndex& index )
     case 1:
       QgisApp::instance()->attributeTable();
       break;
+    case 2:
+      mapStyleDock( true );
+      break;
     default:
       break;
   }

src/ui/qgsoptionsbase.ui

@@ -2919,6 +2919,11 @@
                         <string>Open attribute table</string>
                        </property>
                       </item>
+                      <item>
+                       <property name="text">
+                        <string>Open map styling dock</string>
+                       </property>
+                      </item>
                      </widget>
                     </item>
                    </layout>