Added drop trigger

Hi,

I was doing some tests with this, and I noticed that 'drop something' does not support dropping triggers. At first I tried to add it to the DropType, but then I noticed the drop trigger requires a extra 'on table name'.

Is this code ok?
The part I don't really get is sem_Statement_DropTrigger, which I just copied from sem_Statment_DropSomething.

Thanks!