messageTray: unset this._clickedSummaryItem if we are hiding the summary box pointer...

messageTray: unset this._clickedSummaryItem if we are hiding the summary box pointer and don't have a new clicked summary item

This ensures that this._clickedSummaryItem is always unset correctly.

Because we disconnect the signals that have _adjustSummaryBoxPointerPosition()
as a callback when unsetting this._clickedSummaryItem, we no longer call
setPosition() on this._summaryBoxPointer after it is hidden. Calling
setPosition() shows the box pointer again, which previously resulted in
an empty box pointer staying behind when a notification associated with
a tray icon was clicked.

https://bugzilla.gnome.org/show_bug.cgi?id=659862