Move show labels in kebab menu of child items and linked items widget

What does this MR do and why?

This MR does moves the Show labels toggle to be within the current kebab menu on the Child items and Linked items widget.

To achieve that we have

• Replaced the work_item_tree_actions.vue component with a newly created shared component work_item_more_actions.vue
• Use the shared component work_item_more_actions.vue in
  • Child items widget for Objectives and work item Epics - work_item_tree.vue
  • Child items widget for Issues - work_item_links.vue
  • Linked items widget for all work items - work_item_relationships.vue

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Work item type	Before	After
Epic	Screen Recording 2024-08-06 at 11.23.43 PM.mov	Screen Recording 2024-08-06 at 11.19.28 PM.mov
Objective	Screen Recording 2024-08-06 at 11.24.29 PM.mov	Screen Recording 2024-08-06 at 11.20.51 PM.mov

How to set up and validate locally

Prerequisites:

1. Enable the OKRs feature as mentioned in the OKRs docs (e.g. ::Feature.enabled?(:okrs_mvc, Project.find(<PROJECT_ID>)) and the namespace must have Ultimate plan).
2. Enable Epic work items
   1. Feature.enable(:work_item_epics_rollout)
   2. Feature.enable(:work_item_epics)

Steps to verify at Group level:

1. Login with any user
2. Go to Group > Epics
3. Open or create an Epic
4. Create a child item or add an existing child item in Child items widget
5. Link few items in Linked items widget
6. Add few labels to the added work items
7. Notice the Show labels toggle in the kebab menu instead of Child items / Linked items widget's header
8. Verify if the View Roadmap button is working

Steps to verify at Project level:

1. Login with any user
2. Go to Project > Issues
3. Open or create an Issue/Objective
4. Create a child item or add an existing child item in Child items widget
5. Link few items in Linked items widget
6. Add few labels to the added work items
7. Notice the Show labels toggle in the kebab menu instead of Child items / Linked items widget's header

Related to #463654