Refactor image pool widget

[angelsk]

Currently it's all very tied into sf_image_pool_ids all the ids and js stuff is hardcoded. Not possible to include the image pool chooser as a single widget - allowing you to link several image pool images to a specific model, rather than a group of images with a single featured one.

E.g: A product with a preview image and a large image - each of those should contain an image_pool_image_id and be chosen via the image pool image chooser.

[angelsk]

This is mostly done not - refactored javascript and id => class to allow for multiple usage.

"Selected images" still relies on the image poolable.