Avoid popup the palette while a dragging in the Journal - SL #3999

This is was a pending issue. See more information in the ticket
Note than drag and drop is allowed only with the mouse, not with touch.
Fixes #3999.

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
Merge branch 'fix_3999-try2' of git://github.com/godiard/sugar into godiard-fix_3999-try2