clean removal of dialog windows

[Matthew <matthew slux corepub com>]

I am trying to learn gtk from the tutorial
I have been able to make multiple windows, but I don't know how to remove
them, could someone show an example kill_only_this_window function to do
this? or explain why a simple function like that won't work.
I tried to look through the gimp source, but I don't have enough knowledge
of it to really figure out what is going on, I noticed that widget=NULL
will remove it, but that doesn't look like a proper way to do it.