[ticket/12598] Change predefined searches to drop-down menu

Rebased after #2469

PHPBB3-12598
[ticket/12598] Add icons and lang vars

PHPBB3-12598
[ticket/12598] Shortened lang vars, added new "gear" icon, IE fixes, responsive

PHPBB3-12598
[ticket/12598] Added :hover styling for gradient-icon combination

PHPBB3-12598
[ticket/12598] Various responsive and browser fixes

PHPBB3-12598
[ticket/12598] Add template events search_tools_list_before/after

PHPBB3-12598
[ticket/12598] Bidi border-color fixes

PHPBB3-12598
[ticket/12598] Revert index modifications

PHPBB3-12598
[ticket/12598] Moved self-search from header to index links list

PHPBB3-12598
[ticket/12598] Update events.md from b4 to b5

PHPBB3-12598
Merge 12598