feat: 2424 - new "select existing images" button

[monsieurtanuki]

New files:

• background_task_crop.dart: Background task about product image crop from existing file.
• uploaded_image_gallery.dart: Gallery of all images already uploaded, about a given product.

Deleted file:

• smooth_images_sliver_grid.dart

Impacted files:

• abstract_background_task.dart: added reference to new "crop" task
• app_en.arb: added 4 labels about "existing images" (button, page title, downloading, nothing found)
• image_crop_page.dart: refactored moving code here from ProductImageViewer
• new_crop_page.dart: now we can handle "already existing" images, in addition to the current choice for e.g. NUTRITION_FR
• operation_type.dart: added reference to new "crop" task
• product_image_swipeable_view.dart: unrelated fix - the title of the first reached page was always the title of the first page to the left
• product_image_viewer.dart: added a "select existing image" button; moved code to image_crop_page.dart
• pubspec.lock: wtf
• pubspec.yaml: upgraded off-dart in order to user new methods

What

• New "select existing images" button.
• From there you open a new "existing images" gallery, latest images first.
• Clicking on an image from the gallery, you land on the crop page.
• The "confirm" button on the crop page sets the product / imageField / language image as the crop of the existing image, without any upload (which is much quicker)

Screenshot

new button	new gallery

Fixes bug(s)

• Closes Add image selection from server in the Photo Manager (the expanded view accessible from the carousel) #2424

Part of

• Improve the Photo Manager using new features from openfoodfacts-dart #766

[teolemon]

Resolving dependencies...
Because smooth_app depends on openfoodfacts from path which doesn't exist (could not find package openfoodfacts at "../../../openfoodfacts-dart"), version solving failed.
pub finished with exit code 66

[monsieurtanuki]

Resolving dependencies...

@teolemon It was just a draft as I needed off-dart 2.2.0. I didn't want my code to be lost locally if I or my macbook die - whichever comes first. Looks like we're both OK ;)

Next step is "this image was computed from that uploaded image and those crop parameters, so if I edit it I should see the uploaded image and the crop parameters, not directly the cropped image".

But first, this PR.

[monsieurtanuki]

Thank you @teolemon for the review!
I think we'll be done with most "new" image features by the end of the week.
Then, some UX-refining.