Plugin view: Fix sorting

Broke after moving to Utils::TreeModel. Use QSortFilterProxyModel on top
of TreeModel (can later be used for filtering as well).
Additionally we need to make sure to report changes only for the changed
column now, because otherwise QSortFilterProxyModel thinks that the
change could make re-sorting necessary, and does that in a non-stable
way.

Change-Id: I9fd12c55a45aba4c05f8e318ae8ea9a4ab9f3310
Task-number: QTCREATORBUG-14107
Reviewed-by: Robert Loehning <robert.loehning@theqtcompany.com>
Reviewed-by: hjk <hjk@theqtcompany.com>

src/libs/extensionsystem/pluginview.h

@@ -37,6 +37,10 @@
 #include <QSet>
 #include <QHash>
 
+QT_BEGIN_NAMESPACE
+class QSortFilterProxyModel;
+QT_END_NAMESPACE
+
 namespace Utils {
 class TreeItem;
 class TreeModel;
@@ -71,6 +75,7 @@ private:
 
     Utils::TreeView *m_categoryView;
     Utils::TreeModel *m_model;
+    QSortFilterProxyModel *m_sortModel;
 
     friend class CollectionItem;
     friend class PluginItem;