Allow to copy all the trackers with a keyboard shortcut

If multiple trackers are selected, Ctrl+C will copy only one of them
while the context menu will copy all of them. Fix this inconsistency.

Closes #2675.

src/gui/properties/trackerlist.h

@@ -55,6 +55,7 @@ private:
   QTreeWidgetItem* lsd_item;
   QShortcut *editHotkey;
   QShortcut *deleteHotkey;
+  QShortcut *copyHotkey;
 
 public:
   TrackerList(PropertiesWidget *properties);