[ticket/12672] Make tab intercept; CB for keypress only react without key mods

Changed the callback of keypress inside the <textarea> for the posting
such that, if key 9 is pressed (tab character) and ctrl, shift, alt and meta
are not pressed the tab is placed. Otherwise, nothing is made and default is
not prevented.

https://tracker.phpbb.com/browse/PHPBB3-12672
@nickvergessen the event object is owned by the browser. I don't even know if there's such translation.
By the way, the number 9 is not magic. It is defined as that button on the most left on the 3rd row. AFAIK that's always the tab.