add progress dialog for media delete

aqt/main.py

@@ -1148,10 +1148,21 @@ will be lost. Continue?"""))
             _("Delete unused media?")):
             return
         mdir = self.col.media.dir()
-        for f in unused:
-            path = os.path.join(mdir, f)
-            if os.path.exists(path):
-                send2trash(path)
+        self.progress.start(immediate=True)
+        try:
+            lastProgress = 0
+            for c, f in enumerate(unused):
+                path = os.path.join(mdir, f)
+                if os.path.exists(path):
+                    send2trash(path)
+
+                now = time.time()
+                if now - lastProgress >= 0.3:
+                    lastProgress = now
+                    label = _("Deleted %s files...") % (c+1)
+                    self.progress.update(label)
+        finally:
+            self.progress.finish()
         tooltip(_("Deleted."))
         diag.close()