Add image sizes and the `<picture>` element

As discussed in #6932

### Remaining to-dos:

* [x] Modify all core templates using images, to use the new picture template.
* [x] Add a user interface to select the important part of the image.
* [x] Create a beautiful image sizes icon.
* [x] Translate all messages, done for german and english.
* [x] Theme export and import (including transformation of IDs in imageSize columns)
* [x] Disable with and height input fields in the image size widget if an image size is selected
* [x] Remove all static methods in Image class, make it use object methods, see Toflars PRs madeyourday/contao-core#2 and madeyourday/contao-library#1
* [x] Fix breadcrumbs for `tl_image_size_item`
* [x] Merge SVG support changes (15201d0aa22ae777dd977bd368f49f849a64ee83 and more)
* [x] Add important part support for SVG images

### To discuss:

* [x] What is the right prefix for the `picture` template? `block_` doesn’t seem right.
* [x] Should we make the picturefill polyfill optional? How?
* [x] Should we really remove the `src` attribute from the `<img>` element, as [recommended by picturefill](http://scottjehl.github.io/picturefill/#support)?
* [x] 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?