[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
Probably a good idea. Merging this anyway.