containmentinterface: fix new panel applet order

appletAdded need to relay (-1, -1) to the panel when adding a new preset panel to let the applet append to the last position.

See also: https://invent.kde.org/plasma/plasma-desktop/-/blob/0d133cf082e21eed54788c511b14754337475016/containments/panel/contents/ui/LayoutManager.js#L47

Panels before plasma-desktop@0d133cf0 are not affected whether this MR is merged or not.