use a timer to automatically show progress window

We were previously relying on the DB progress hook to cause the
progress window to display.

Qt's progress dialogs do have built in support for automatically
showing, but it's easier to add a timer than change the existing
code to use it.

qt/aqt/progress.py

@@ -25,6 +25,7 @@ class ProgressManager:
         self.app = QApplication.instance()
         self.inDB = False
         self.blockUpdates = False
+        self._show_timer: Optional[QTimer] = None
         self._win = None
         self._levels = 0
 
@@ -114,6 +115,10 @@ class ProgressManager:
         self._firstTime = time.time()
         self._lastUpdate = time.time()
         self._updating = False
+        self._show_timer = QTimer(self.mw)
+        self._show_timer.setSingleShot(True)
+        self._show_timer.start(600)
+        self._show_timer.timeout.connect(self._on_show_timer)
         return self._win
 
     def update(self, label=None, value=None, process=True, maybeShow=True):
@@ -143,6 +148,9 @@ class ProgressManager:
             if self._win:
                 self._closeWin()
             self._unsetBusy()
+            if self._show_timer:
+                self._show_timer.stop()
+                self._show_timer = None
 
     def clear(self):
         "Restore the interface after an error."
@@ -189,6 +197,10 @@ class ProgressManager:
         "True if processing."
         return self._levels
 
+    def _on_show_timer(self):
+        self._show_timer = None
+        self._showWin()
+
 
 class ProgressDialog(QDialog):
     def __init__(self, parent):