Visualization: Branching Support

[kahboom]

Description

This is an EPIC to add support for branching in the Visualization.

Depends on #193 and #270 <- this must be complete before branching can be fully implemented, though development can start before then.

Business Requirements

• Branches can be started from any step (e.g. START, MIDDLE, END)
• There can be more than one branch from a single step
  • For example, with a CHOICE step, it's like if/else, so there will automatically be two branches, depending on the outcome of the condition
  • We don't yet support a "default flow" and "alternate flow", so the branches should each be equal weight in priority
• Branches contain steps, and those steps can be type MIDDLE or END
• If it's of type MIDDLE, then it should be merged back
  • Maria: It can merge back to any step, MIDDLE and END (well, I guess not START)
  • Just merged to the following step to that EIP unless it finishes in an END
  • Though not the most common scenario, it should be possible to have one branch that ends in END, and one branch that ends in MIDDLE and gets merged back

Tasks

• Branching: Support basic EIP branching #782
• Branching: Multi-branch support #785
• Branching: Ability to view and edit branch step properties #784
• Branching: Insert/replace/delete/append actions for branch steps #783
• Branching: Show branches even if empty #1158
• Branching: Build branches visually #786
• Branching: Ability to delete branches in canvas #1240