Added the special deletion. Pressing backspace, the user has a normal deletion

Added the special deletion. Pressing backspace, the user has a normal deletion
(which can be either graphical or textual). Pressing backspace + alt, the user
has a special deletion (which can be either textual or graphical).
To implement this feature, all parser's methods concerning the deletion have
been modified: now, all of them have a boolean parameter, which indicates which
kind of deletion the method has to operate.
All methods' names have been changed: the word gdelete has been substitued with
drop.