[BUGFIX] Use page layout specific column labels

Labels of columns in page layouts were only translatable with the generic
non-specific key "flux.grid.columns.$columnName".

This patch sets the page layout's grid parent object to be the page layout's form,
so that the resolved parent ID is that of the form.

A column named "one" within a page layout with id "twocol" will thus get
the label key "flux.twocol.columns.one", just as it was for flux 8.

Resolves: #1468

Classes/Provider/AbstractProvider.php

@@ -342,11 +342,15 @@ public function getGrid(array $row)
                         $gridColumn->setColSpan($object['colspan'] ?? 1);
                     }
                 }
+                $grid->setParent($form);
                 return $grid;
             }
         }
         $grid = $this->grid ?? $this->extractConfiguration($row, 'grids')['grid'] ?? Grid::create();
         $grid->setExtensionName($grid->getExtensionName() ?: $this->getControllerExtensionKeyFromRecord($row));
+        if ($form) {
+            $grid->setParent($form);
+        }
         return $grid;
     }