Fix closing the documents when quitting

When quitting we avoid doing some unnecessary actions, and used to
simply destroy the Scintilla widget (and thus the notebook page)
instead of the elaborate UI updates.

Unfortunately, when the infobars landed they changed what is packed as
a notebook page, and now destroying the Scintilla widget alone is not
enough to close the page.  Fix this by properly removing the whole page
no matter what it contains.

This issue was visible when quitting Geany with a project open.