Wrong return type in ImageInterface::getTookit docblock.

Problem/Motivation

The documentation of ImageInterface::getTookit lists the wrong return type.

  /**
   * Returns the image toolkit used for this image file.
   *
   * @return string
   *   The image toolkit.
   */
  public function getToolkit();

With this, IDE's can't make proper suggestions for the returned value.

Proposed resolution

Change string to \Drupal\Core\ImageToolkit\ImageToolkitInterface.

Remaining tasks

none

API changes

none