handle blur in preference window

preferencemanager.js

@@ -21,6 +21,9 @@ function showPreferences_int(){
         // when you should delete the corresponding element.
         preferencesWindow = null
       })
+      preferencesWindow.on('blur',() => {
+        preferencesWindow.close();
+      })
 }
 }
 module.exports.showPreferences = function(){