Branching: Insert/replace/delete/append actions for branch steps

[kahboom]

Please describe the feature that you want to propose

At the moment, we are only rendering branches, but you are not able to make any changes to the steps within the branch. We should allow the same basic functions for "normal" steps as for the branch steps.

Epic #140

Tasks

• Add ability to insert a step between two branch steps #1114
• Add ability to delete a branch step #1115
• Add ability to replace a branch step #1116
• Add ability to append a branch step #1117
• Branching: Add ability to prepend steps #1161