Hi Bart,
You can do a long press on the "Add gobo slot", this give you the dialog to add multiple slots at once.
I must warn you though, adding 256 slots does not really work everywhere, last time I did something like that I made 127 in the first gobo wheel and 128 in the second.
For editing the XML I recommend using something like Notepad++ to make it easier to keep the XML file valid.
Greetings,
Bootsy