"Drag-and-drop" pipeline creation screen

Create a new feature where pipelines can be created without actually having to run plugin instances.

We should explore a "drag-and-drop" user interface. A side panel contains plugins which are dragged into the central workspace. Parent-child relationships between nodes should be determined based on the location of where plugins are dragged to. For example, if plugin B is dragged underneath plugin A and plugin C, and plugin B is closer to A than C, then B should be after A in the pipeline.

Clicking on a plugin in the workspace opens a menu where the default parameters for a plugin can be edited.

The issue names no files, tests, or entry points. Start by locating the existing pipeline creation UI and plugin-instance flow, then determine how a drag-and-drop workspace, plugin parameter editing, and parent-child ordering would fit together. Done means pipelines can be composed without running plugin instances and their relationships and default parameters can be edited as described.

Written by the indexing model from the issue text.