Fix flashing when opening the stats, empty cards or find dupes dialogs

[iamllama]

There's a slight visual regression (sorry!) after #3925 whereby the aforementioned dialogs flash more(?) after opening

AnkiWebView.set_kind now potentially sets a new page, and it flashes despite setting its background colour immediately after. set_kind needs to be called for those dialogs because their forms are codegen'd from qt ui designer files, which, as far as i can tell from the schema, don't seem to allow passing in custom arguments directly to widget constructors

The fix proposed here is to replace the webview widgets in those ui files with subclasses that have the correct kind filled in, to avoid calling set_kind after init

After this, only the legacy deck stats would still call it (and use the hack in set_kind), as it shares the same form (and webview widget) with the "new" deck stats

EDIT: I've overthought this by overlooking that the page's background colour wasn't set again after set_kind sets a new page, adding it seems to have gotten rid of the flashes

[dae]

Simple solutions are always nice! Thank you both for the quick fix.