a11y: Add block quote role
Add a new GTK_ACCESSIBLE_ROLE_BLOCK_QUOTE role for block quotes/block quotations.
ARIA has a corresponding "blockquote" role as well. [1]
The role is used e.g. in document editors like LibreOffice or web browsers like Firefox.
According to the ARIA spec (§ 5.2.8.4, [2]), the blockquote role is among those that can be named by the author, and the superclass role is section. [1]
Related change for LibreOffice making use of the new role: [3]
[1] https://www.w3.org/TR/wai-aria-1.2/#blockquote [2] https://www.w3.org/TR/wai-aria-1.2/#namefromauthor [3] https://gerrit.libreoffice.org/c/core/+/158685