Fix crash on cancelling the open-element dialog before collection load finishes

Bugtracker #291: clicking Cancel on the open/save-element dialog before
the user collection finishes loading crashes the whole application with
an unhandled pointer exception.

ElementsCollectionModel::loadCollections() loads collections in the
background via QtConcurrent::map(m_items_list_to_setUp, setUpData) -
worker threads call setUpData() on each ElementCollectionItem
(a QStandardItem), which does setFlags()/setData() on it.

ElementDialog::execConfiguredDialog() deletes the dialog immediately
after exec() returns:

  element_dialog->exec();
  ...
  delete element_dialog;

That destroys the tree view and its ElementsCollectionModel, which as
a QStandardItemModel frees all its items in its destructor. Nothing
waited for the QtConcurrent::map() to finish first, so on Cancel before
loading completes, background threads were still calling setUpData()
on items the main thread had just freed - a use-after-free race.

Add an ElementsCollectionModel destructor that waits for the future
before QStandardItemModel's destructor runs. QFuture::waitForFinished()
on a default-constructed (never-started) future returns immediately, so
this is a no-op whenever loading already completed - the crash path is
the only one affected.

sources/ElementsCollection/elementscollectionmodel.h

@@ -35,6 +35,7 @@ class ElementsCollectionModel : public QStandardItemModel
 
 	public:
 		ElementsCollectionModel(QObject *parent = Q_NULLPTR);
+		~ElementsCollectionModel() override;
 
 		QVariant data(const QModelIndex &index, int role) const override;
 		QMimeData *mimeData(const QModelIndexList &indexes) const override;