implemented sorting in project explorer

Wow I can't believe I forgot about the QSortFilterProxyModel
for like 1.5 years straight.

app/widget/projectexplorer/projectexplorerlistviewbase.cpp

@@ -42,11 +42,14 @@ ProjectExplorerListViewBase::ProjectExplorerListViewBase(QWidget *parent) :
 
 void ProjectExplorerListViewBase::mouseDoubleClickEvent(QMouseEvent *event)
 {
+  // Cache here so if the index becomes invalid after the base call, we still know the truth
+  bool item_at_location = indexAt(event->pos()).isValid();
+
   // Perform default double click functions
   QListView::mouseDoubleClickEvent(event);
 
   // QAbstractItemView already has a doubleClicked() signal, but we emit another here for double clicking empty space
-  if (!indexAt(event->pos()).isValid()) {
+  if (!item_at_location) {
     emit DoubleClickedEmptyArea();
   }
 }