Add section headers to plus button dropdown #41128

What does this MR do?

Adds section header to plus button dropdown in repository next to file path breadcrumb.

Are there points in the code the reviewer needs to double check?

No. Just check with @jramsay, @sarrahvesselov and @dimitrieh if this closes #41128 (moved) or leaves it open for further discussion.

Why was this MR needed?

Consider this fix as a middle step to visually improve and promote repository actions and like New branch and New tag.

Screenshots

BEFORE	AFTER

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the style guides

What are the relevant issue numbers?

#41128 (moved)