Speed up lookup time

By adding another variable we can get O(1) lookup time instead of O(n).
Fix up 5f415c292d.

src/gui/properties/peerlistwidget.h

@@ -30,6 +30,7 @@
 #define PEERLISTWIDGET_H
 
 #include <QHash>
+#include <QSet>
 #include <QTreeView>
 
 class QHostAddress;
@@ -85,6 +86,7 @@ private:
     PropertiesWidget *m_properties = nullptr;
     Net::ReverseResolution *m_resolver = nullptr;
     QHash<PeerEndpoint, QStandardItem *> m_peerItems;
+    QHash<QHostAddress, QSet<QStandardItem *>> m_itemsByIP;  // must be kept in sync with `m_peerItems`
     bool m_resolveCountries;
 };