I can edit my sequential declarative pipeline stages in the pipeline visualization plugin

For Declarative 1.3 (JENKINS-46809), we're adding support for sequential groups of stages, either as part of a set of parallel stages or on its own in a top-level stage. I won't bother going into all the rationale behind it. =) See the Declarative PR for the relevant changes, examples, discussion, etc.

This will require changes for both visualization and the editor:

• Visualization - see my pathetic attempts at mockups attached here. jamesdumay probably will come up with something better in like five seconds. =)
• Editor - Should be something like we have for parallel stages - reordering would be good too.