Enhance `djangocms-picture` so that it can handle all pictures (and picture plugins)

[mrbazzan]

The plan is to create a PictureField and PictureFormField that let's you select an image,
potentially define its format, size, ...
Those fields would be reusable in custom models or forms.

According to fsbraun,

... we are planning to give djangocms-picture a boost so that it can be used:

• With multiple backends (e.g., filer, but also other solutions, like digital asset management)
• With reusable Picture field

The objective is that all pictures can be handled by djangocms-picture and that other picture plugins (like djangocms-frontend's) actually depend and reuse picture's capabilities. Any support is welcome.

Originally posted by @fsbraun in #140 (comment)