[ticket/11241] Add dropdown control button.

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
[ticket/11241] Add border to dropdown menu options.

PHPBB3-11241
[ticket/11241] Fix the disappearing dropdown when responsive changes take effect

PHPBB3-11241
[ticket/11241] Retain button gradient when menu is active.

PHPBB3-11241
Merge remote-tracking branch 'prototech/ticket/11241' into develop

* prototech/ticket/11241:
  [ticket/11241] Retain button gradient when menu is active.
  [ticket/11241] Fix the disappearing dropdown when responsive changes take effect
  [ticket/11241] Add border to dropdown menu options.
  [ticket/11241] Add events & var for extensions to dropdown.
  [ticket/11241] Fix the dropdown in RTL languages.
  [ticket/11241] Use DEFINE for the block.
  [ticket/11241] Consolidate topic actions into "Topic tools" dropdown.
  [ticket/11241] Add dropdown control button.