Popups should emit a closed() signal when closed

[Alberto Mardegan]

Currently, Popups run an animation when closed. If the caller destroyes the popup (or the UI which invokes it) immediately after calling PopupUtils.close(), the QML engine runs into an error because the animation is still running. See bug 1315341 for an example.

The Popup should emit a closed() signal when all of its animations have completed and it's safe to destroy it. We are currently working around this by listening to a change on the "visible" property, which in the current Popup implementation is set to "false" at the end of the closing animation; but this is fragile because it depends on an undocumented implementation detail.