add support for custom images (selfservice only)

[Fosca42]

This adds UI support for custom images:

• image files can be dragged upon a drop zone and will be added to custom image list
• custom images can be selected by simply clicking them, they get selected in the UI and should appear in the visualization
• custom images can be removed by clicking the trash can icon on their lower right corner

So, this uses all 5 custom image API calls:

• getCustomImageFiles (/custom-images/[shoppingCartItemId]/[customImageId]/get-custom-image-file-list "GET")
• deleteCustomImageFile (/custom-images/[shoppingCartItemId]/[customImageId]?file=[file] "DELETE")
• uploadCustomImageFile (/custom-images/[shoppingCartItemId]/[customImageId]?file=[file] "POST")
• selectCustomImage (/custom-images/[shoppingCartItemId]/[customImageId]?file=[file] "PUT")
• getSelectedCustomImages (/custom-images/[shoppingCartItemId]/get-selected-custom-images "GET")