containments/panel: avoid placing the leftmost/topmost applet at the end

childAt won't find any applet when adding a new preset panel
because all applets don't have a valid width during initialization.
But when replacing the leftmost/topmost widget with an alternative,
`childAt` also can't find any applet. The difference is when replacing,
layout's width is larger than 0 because other applets have valid
widths.

BUG: 453998
FIXED-IN: 5.25