Reasons for this fix:

1. this was clearly dumb logic;
2. this is why we see the available pages collapse when we try to move

items using the quick move.

For (2) the following was happening:

• in ApplicationComponents::onMoveItemsRequested we requested the pageListModel property;
• in Available*PagesModel::pageListModel we set the model on the sort proxy each time;
• the sort proxy didn't check we didn't really change the model (I'd argue that's a bug in QSortFilterProxyModel to be honest);
• since it thought the model changed, it issued a modelReset() signal;
• the tree view in AvailablePagesView subsequently collapsed everything (which makes sense at that point).