Add image sizes and the `<picture>` element

As discussed in #6932

### Remaining to-dos:

* [ ] Modify all core templates using images, to use the new picture template.
* [ ] Add a user interface to select the important part of the image.
* [ ] Create a beautiful image sizes icon.
* [ ] Translate all messages.

### To discuss:

* [ ] What is the right prefix for the `picture` template? `block_` doesn’t seem right.
* [ ] Should we make the picturefill polyfill optional? How?
* [ ] Should we really remove the `src` attribute from the `<img>` element, as [recommended by picturefill](http://scottjehl.github.io/picturefill/#support)?
* [ ] Issue #2426 wasn't solved correctly. The size of the PNG palette is taken from the source image, which is wrong. The number of colors of the generated image has to be used instead. Is my fix for this issue OK?