fix: stop thumbnail jobs when closing library (TagStudioDev#583)

tagstudio/src/qt/ts_qt.py

@@ -586,6 +586,7 @@ def close_library(self, is_shutdown: bool = False):
 
         self.lib.close()
 
+        self.thumb_job_queue.queue.clear()
         if is_shutdown:
             # no need to do other things on shutdown
             return