Fix flashing when opening the stats, empty cards or find dupes dialogs (#3928)

* add AnkiWebView subclasses for stats, empty cards and find dupes ui

* update ui files to use subclassed webviews instead

* remove superfluous calls to AnkiWebView.set_kind

* revert impl

* set page background colour after setPage in AnkiWebView.set_kind

(cherry picked from commit a74fd74)

Author: iamllama

qt/aqt/webview.py

@@ -399,6 +399,7 @@ def set_kind(self, kind: AnkiWebViewKind) -> None:
         # signal from the first one is received
         if kind != AnkiWebViewKind.DEFAULT:
             self.setPage(AnkiWebPage(self._onBridgeCmd, kind, self))
+            self.page().setBackgroundColor(theme_manager.qcolor(colors.CANVAS))
 
     def page(self) -> AnkiWebPage:
         return cast(AnkiWebPage, super().page())