containments/panel: fix applet order in new panel
indexAtCoordinates
returns 0 when x < 0 or y < 0 in the while loop.
But when a new preset panel is added, childAt
will not be able to
find any applet because they are not initialized yet, so the new position
for an applet is always 0, which results in reversed order of applets.
Edited by Fushan Wen