CardView: fix Tweak for choosing drag'n'drop capability

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
CardView: check "groupForDrop" variable before use, in case the group in the center wasn't found

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
Merge pull request #130 from webOS-ports/tofe/work

CardView: fix Tweak for choosing drag'n'drop capability