Introduce a priority thread pool

This pool is meant to be used for threads where the user expects a response as fast as possible.

picard/tagger.py

@@ -160,6 +160,12 @@ class Tagger(QtWidgets.QApplication):
         # It's a valid reference, but its start() method doesn't work.
         self.thread_pool = QtCore.QThreadPool(self)
 
+        # Provide a separate thread pool for operations that should not be
+        # delayed by longer background processing tasks, e.g. because the user
+        # expects instant feedback instead of waiting for a long list of
+        # operations to finish.
+        self.priority_thread_pool = QtCore.QThreadPool(self)
+
         # Use a separate thread pool for file saving, with a thread count of 1,
         # to avoid race conditions in File._save_and_rename.
         self.save_thread_pool = QtCore.QThreadPool(self)
@@ -341,6 +347,7 @@ class Tagger(QtWidgets.QApplication):
         self._acoustid.done()
         self.thread_pool.waitForDone()
         self.save_thread_pool.waitForDone()
+        self.priority_thread_pool.waitForDone()
         self.browser_integration.stop()
         self.webservice.stop()
         self.run_cleanup()

picard/ui/metadatabox.py

@@ -446,7 +446,8 @@ class MetadataBox(QtWidgets.QTableWidget):
             return
         if self.selection_dirty:
             self._update_selection()
-        thread.run_task(self._update_tags, self._update_items)
+        thread.run_task(self._update_tags, self._update_items,
+            thread_pool=self.tagger.priority_thread_pool)
 
     def _update_tags(self):
         self.selection_mutex.lock()