[ticket/11979] Add ability to create dropdown menu to prosilver

PHPBB3-11979
[ticket/11979] Fix errors in dropdown

PHPBB3-11979
Merge remote-tracking branch 'cyberalien/ticket/11979' into develop

* cyberalien/ticket/11979:
  [ticket/11979] Fix errors in dropdown
  [ticket/11979] Add ability to create dropdown menu to prosilver