Support more Plupload widgets

Problem/Motivation

Plupload supports different widgets, but we currently implement only "Plupload queue". We should allow usage of different widgets via D8 plugins. This approach will allow us to support both "Plupload queue" and "jQuery UI" widgets, but there is even more interesting side-effect. We will be able to use Core Plupload API, which will allow people to provide completely customized Plupload experience.

More info and examples: http://plupload.com/examples/

Proposed resolution

Introduce "Plupload widget" plugin type and port current implementation to "Plupload queue" plugin, which will be used by default. Other widget plugins can be implemented later when there is a need for that.

Remaining tasks

User interface changes

API changes