Commit 2310a193 made it impossible to add an item to playlist
by double click in double click mode. This patch restores and tweaks
the original algorithm. (to keep bug 279513 fixed)

Used ideas from a patch by Yichao Yu, but implemented differently.

P.S.: Ralf, event heard about QRect::contains()? :-)

REVIEW: 109162
CCBUG: 279513