Dropdown for selecting header

**Is your feature request related to a problem? Please describe.**
Icons with text like H1, H2, H3 is not user-friendly. The "bigger heading" and "smaller heading" buttons aren't very user-friendly either.

**Describe the solution you'd like**
A toolbar button that when pressed presents the user with a dropdown containing "Header 1", "Header 2", "Header 3", "Header 4", "Header 5" and "Header 6".

Start by locating the editor toolbar implementation and the existing heading-related buttons. Add a dropdown offering Header 1 through Header 6, then verify that each option applies the corresponding heading level in the editor.

Written by the indexing model from the issue text.