fix memory corruption on 'Close the file, discard..'

if you remove some file from the file system
that is open in a KTextEditor document, some
message will show up

if you click the 'Close the file, discarding its content.'
button, we unfortunately delete "this" if the container
application like Kate provides the necessary interface

delay this, like e.g. the vim commands do

(+ guard vim commands by passing context pointer to doc/view)

BUG: 433180