Separate files for PluginRegistrationDialog...

... so that PluginManager can avoid using GUI

src/menus/PluginMenus.cpp

@@ -6,6 +6,7 @@
 #include "../CommonCommandFlags.h"
 #include "../Menus.h"
 #include "../PluginManager.h"
+#include "../PluginRegistrationDialog.h"
 #include "../Prefs.h"
 #include "../Project.h"
 #include "../ProjectSettings.h"
@@ -36,10 +37,20 @@ AudacityProject::AttachedWindows::RegisteredFactory sMacrosWindowKey{
    }
 };
 
+bool ShowManager(
+   PluginManager &pm, wxWindow *parent, EffectType type)
+{
+   pm.CheckForUpdates();
+
+   PluginRegistrationDialog dlg(parent, type);
+   return dlg.ShowModal() == wxID_OK;
+}
+
 void DoManagePluginsMenu(AudacityProject &project, EffectType type)
 {
    auto &window = GetProjectFrame( project );
-   if (PluginManager::Get().ShowManager(&window, type))
+   auto &pm = PluginManager::Get();
+   if (ShowManager(pm, &window, type))
       MenuCreator::RebuildAllMenuBars();
 }