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.hpp

@@ -52,7 +52,6 @@ public:
                                                    MessageHandler  on_message,
                                                    CloseHandler    on_close,
                                                    CloseHandler    on_destroyed);
-    static void show_modeless_dialog(PluginWebDialog* dialog);
     static void post_message(PluginWebDialog* dialog, const nlohmann::json& data);
     static void request_close(PluginWebDialog* dialog);