containments/panel: iterate all items between start index and end index when pressed
This fixes other applets jumping around when user tries to move one applet,
because MouseArea doesn't send all mouse events, so the current item
fetched from childAt can miss some applets when mouse is moving very
fast.
| Before | After |
|---|---|
| 2022-09-29_22-44-35 | world_best_panel |
Edited by Fushan Wen