Fix drag and drop error

Fix drag and drop error

Linus noted an odd "CRITICAL" warning when ripping off a page of the
notebook and then dropping it within the same notebook.

Turns out we need to simply accept a drop on ourselves and gtk does the
rest correctly.

I also fixed the fact that we incorrectly declared the callback as 'void'.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>