Improved multi case initial reading

When creating a multi case group from multiple files, read grid from first file.
The main grid from the first case is reused for the other cases.
NOTE: Metadata and well information is not read for the other cases
p4#: 20655

ApplicationCode/UserInterface/RIMainWindow.cpp

@@ -1312,10 +1312,5 @@ void RIMainWindow::slotOpenMultipleCases()
         appendEGRIDFilesRecursively(fileName, gridFileNames);
     }
 
-    for (int i = 0; i < gridFileNames.size(); i++)
-    {
-        QString fileName = gridFileNames[i];
-
-        app->addEclipseCase(fileName);
-    }
+    app->addEclipseCases(gridFileNames);
 }