Create plugin windows off the webview callback stack

wx 3.3.2 delivers webview script messages synchronously inside the native
callback on GTK and macOS, so script plugins run with the plugins-dialog
webview's signal/delegate frame on the stack. Creating and presenting the
orca.host.ui window from there crashed on Linux at Raise() --
gtk_window_present while GTK's deferred show was still in flight.

Defer the whole window creation to a CallAfter with a pre-bound registry
handle (post/close stay FIFO-safe, teardown races become a no-op), and
drop Raise() plus the show_modeless_dialog wrapper: Show() already
activates and fronts a new window on every platform.

src/slic3r/GUI/PluginWebDialog.cpp

@@ -163,14 +163,6 @@ PluginWebDialog* PluginWebDialog::create_modeless_dialog(wxWindow*       parent,
                                std::move(on_destroyed));
 }
 
-void PluginWebDialog::show_modeless_dialog(PluginWebDialog* dialog)
-{
-    if (dialog == nullptr)
-        return;
-    dialog->Show();
-    dialog->Raise();
-}
-
 void PluginWebDialog::post_message(PluginWebDialog* dialog, const nlohmann::json& data)
 {
     if (dialog != nullptr && dialog->is_open())