[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/12559] Add forum setting to limit subforum legend

Squashed all commits, they were basically just typos and errors

PHPBB3-12559
[ticket/12559] Applied DeMorgan and removed unnecessary true:false

PHPBB3-12559
[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/12559] Updated copyright information

PHPBB3-12559
[ticket/12598] Update events.md from b4 to b5

PHPBB3-12598
[ticket/12598] Close dropdown on inputbox focus

And change webkit-search-cancel-button cursor to pointer

PHPBB3-12598
[ticket/12598] Remove dropdown

:'(

PHPBB3-12598
[ticket/12598] Move adv-search link into form

Now that we don't have a dropdown anymore, this is cleaner.

PHPBB3-12598
[ticket/12598] Minor RTL fix

PHPBB3-12598
[ticket/12654] Improve header searchbox

PHPBB3-12654
[ticket/12598] Change predefined searches to drop-down menu

Rebased after #2469

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

PHPBB3-12598