lua: implement internal.trigger in C

The patch rewrites Lua implementation of trigger list in C as a wrapper
over list of lbox triggers.

Backward compatibility is a bit broken after this patch. Firslty, despite
usually all the triggers are inserted to the beginning of the trigger list,
trigger list, implemented in Lua, inserted new triggers at the end. The
patch fixes this inconsistency. Also, triggers using lbox_trigger_reset
return nil when a trigger is deleted, and triggers using Lua implementation
return removed trigger in this case. The behavior is unified now - all
triggers return nil on deletion.

Part of #6484
Part of #8659

NO_CHANGELOG=internal
NO_DOC=internal