autoclose: Refactor so that "document-close" is not necessary.

The document-close handler was just to free data and disconnect handlers
associated to the sci widget. The former can be simplified with
g_object_set_data_full() and the signal handler will be disconnected anyway
on widget destroy.
Merge pull request #2 from kugel-/autoclose-v0.2

Autoclose v0.2
Merge pull request #126 from scriptum/master

autoclose: Refactor so that "document-close" is not necessary