Resolve "Frequent Items Initialisations can be optimised and deferred in idleCallback"

What does this MR do?

Moves the initialisation of the frequent item List to a requestIdleCallback as it anyhow is behind a submenu. Adds also a check if someone already opened the dropdown and then renders it. Cuts down the timing for each page DomContentLoaded from 50 to 21ms on an average run.

Before

After

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #56328 (closed)