[Feature]: Structural edits: add, move and delete requirements; edit traces  #83

What type of issue is this?

• Bug report
• Feature request

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the reqifviewer
• I have searched open and closed issues to ensure it has not already been reported

Description

Acceptance criteria

• Add a requirement of a chosen spec type at a position in a specification; move it (drag in the outline); delete it (and decide what happens to its traces).
• Add and remove traces between requirements with a relation type.

Steps to Reproduce

NA

System Configuration

• reqifviewer version: 6.1.0 (development, with [Refactor] Blazor Blueprint app shell and Blazor Web App hosting #56)
• Environment (Operating system, version and so on): Windows 11, Chrome
• .NET Framework version: .NET 10
• Additional information: Plan item E-05 of the viewer improvement plan. Size: L. Depends on: [Feature]: Edit session with change tracking and undo  #81, [Feature]: Helpers to create and change elements with valid identifiers, LastChange and ReqIFContent references  reqifsharp#85, a decision on the editing scope (values only, or also structure).