applets/notifications: Fix notifications jumping around when dialogs close or lose focus

Keep around the last non-notification focused Plasma dialog, so we can maintain
stable notification positioning even if we have a Plasma dialog which loses focus
but stays visible (think pinned System Tray applets).

This also fixes notifications jumping down on close, since now the dialog used to
calculate the notification popups's offset is never itself a NotificationPopup.

BUG: 449085
BUG: 461492
BUG: 426187
FIXED-IN: 5.27