[ticket/11241] Implement dropdown element.

PHPBB3-11241
[ticket/11241] Consolidate topic actions into "Topic tools" dropdown.

PHPBB3-11241
[ticket/11241] Use DEFINE for the block.

PHPBB3-11241
[ticket/11241] Fix the dropdown in RTL languages.

PHPBB3-11241
[ticket/11241] Add events & var for extensions to dropdown.

PHPBB3-11241