Fix #159: reset unused-element highlight when elements become used again

ElementsCollectionModel::highlightUnusedElement() only ever painted the
currently-unused elements red; it never cleared the background of items
that were no longer unused. So when an element was re-added to a project
and saved, its red 'unused' highlight persisted until the model was
rebuilt from scratch.

Reset every item's background before re-applying the highlight to the
current unused set.

sources/ElementsCollection/elementscollectionmodel.cpp

@@ -538,6 +538,11 @@ QList<QETProject *> ElementsCollectionModel::project() const
 */
 void ElementsCollectionModel::highlightUnusedElement()
 {
+		//Reset the background of every item first, so elements that are no
+		//longer unused lose their previous red highlight (issue #159).
+	for (ElementCollectionItem *eci : items())
+		eci->setBackground(QBrush());
+
 	QList <ElementsLocation> unused;
 
 	foreach (QETProject *project, m_project_list)