Assets pipeline

This is a long-standing issue (sub task of #126) but maybe easier to implement so it's better if it has its own issue.

It would be nice to have a way to specify an assets pipeline step-by-step dragging-and-dropping elements in a white canvas. Each step/task could be scriptable/customizable/parametrizable (maybe related to #475) and connectable to following steps.

Each task block should have some inputs and outputs, and we should be able to connect only compatible inputs/outputs. The big problem here is that we should be able to use input information (e.g. tag name?) in the task output (e.g. output filename using the tag name).

Related issues: #292, #1998