Reflect CheckForBetaUpdates change w/o restart on Windows

If the user changes their beta updates preference, reflect it
immediately so that subsequent manual check for updates works as
expected.

This already worked correctly on macOS.

src/edapp.cpp

@@ -478,14 +478,11 @@ bool PoeditApp::OnInit()
 #endif // USE_SPARKLE
 
 #ifdef __WXMSW__
-    wxString appcast = "https://poedit.net/updates_v2/win/appcast";
-    if ( CheckForBetaUpdates() )
-    {
-        // Beta versions use unstable feed.
-        appcast = "https://poedit.net/updates_v2/win/appcast/beta";
-    }
-
-    win_sparkle_set_appcast_url(appcast.utf8_str());
+    if (CheckForBetaUpdates())
+        win_sparkle_set_appcast_url("https://poedit.net/updates_v2/win/appcast/beta");
+    else
+        win_sparkle_set_appcast_url("https://poedit.net/updates_v2/win/appcast");
+
     win_sparkle_set_can_shutdown_callback(&PoeditApp::WinSparkle_CanShutdown);
     win_sparkle_set_shutdown_request_callback(&PoeditApp::WinSparkle_Shutdown);
     auto buildnum = GetAppBuildNumber();

src/prefsdlg.cpp

@@ -1135,10 +1135,15 @@ class UpdatesPageWindow : public PrefsPanel
 
     void SaveValues(wxConfigBase& cfg) override
     {
+        cfg.Write("check_for_beta_updates", m_beta->GetValue());
 #ifdef __WXMSW__
         win_sparkle_set_automatic_check_for_updates(m_updates->GetValue());
+        // FIXME: don't duplicate this code from PoeditApp::OnInit()
+        if (m_beta->GetValue())
+            win_sparkle_set_appcast_url("https://poedit.net/updates_v2/win/appcast/beta");
+        else
+            win_sparkle_set_appcast_url("https://poedit.net/updates_v2/win/appcast");
 #endif
-        cfg.Write("check_for_beta_updates", m_beta->GetValue());
 #ifdef USE_SPARKLE
         UserDefaults_SetBoolValue("SUEnableAutomaticChecks", m_updates->GetValue());
 #endif