actions: Restore dialog accept on Enter behavior

Commit 3811371b ("actions: Use xfce_message_dialog_new in confirmation dialogs") rewrote action dialogs to use libxfce4ui functions instead of the GTK ones, however it deleted a call to gtk_dialog_set_default_response (), apparently accidentally.

Due to this, now these actions can no longer be accepted by simply pressing Enter, which is quite annoying and breaks user's muscle memory.

Restore the old behavior by adding that gtk_dialog_set_default_response () call back to the code.

Fixes: 3811371b ("actions: Use xfce_message_dialog_new in confirmation dialogs")