Use strtolower for folders when importing images via the AssetImport

The name of the folder for the images in the _isotope_ folder has to be converted to lowercase when importing images via the AssetImport.php Otherwise, the images are not displayed correctly (e. g., [Backend/Product/Label.php#L46](https://github.com/isotope/core/blob/master/system/modules/isotope/library/Isotope/Backend/Product/Label.php#L46) or [Model/Gallery/Standard.php#L242](https://github.com/isotope/core/blob/master/system/modules/isotope/library/Isotope/Model/Gallery/Standard.php#L242)).