Journal: fix the API for drag and drop - SL #3999

TestCase:

- open Paint activity, draw something
- go to the Journal
- drag the Paint entry to the Documents drive
- insert a USB stick
- drag the Paint entry to the USB drive

The paint should be in Documents and in the stick

Note: this needs the upstream fix
https://bugzilla.gnome.org/show_bug.cgi?id=692844 which is not in GTK+
3.6 but is added in current olpc builds as gtk3-3.6.4.1.fc18.olpc2.
So it can be tested on an XO.

Note: this does not fix the unwanted palette popup as described in the
ticket.  That will be a separate patch.

Signed-off-by: Manuel Quiñones <manuq@laptop.org>