New files:
* `crop_grid.dart`: heavily inspired from package `crop_image` - ideally we should put it back there.
* `crop_helper.dart`: Crop Helper - which crop tool do we use, and the method to use it.
* `new_crop_page.dart`: Page dedicated to image cropping. Pops the resulting file path if relevant.
* `rotated_crop_controller.dart`: heavily inspired from package `crop_image` BUT with the rotation feature - ideally we should put it back there.
* `rotated_crop_image.dart`: heavily inspired from package `crop_image` BUT with the rotation feature - ideally we should put it back there.
* `rotation.dart`: 90 degree rotations - ideally we should put it back in package `crop_image`

Impacted files:
* `image_crop_page.dart`: now relying on new class `CropHelper` in order to get the appropriate crop tool (e.g. old or new)
* `pubspec.lock`: wtf
* `pubspec.yaml`: added package `image` for good performances regarding image encoding
* `user_preferences_dev_mode.dart`: added a "Use new crop tool" switch (default is `false`)

….dart