Make GUI labels optional

[matyalatte]

I made GUI labels optional because they were used only when the "gui" array had multiple definitions. You don't need to define "label" for each command now.

{
    "gui": {
        "command": "echo Hello!",
        "components": []
    }
}

Also, you can now use "window_name" as the default value of "label".

{
    "gui": [
        {
            // The value of "label" also become "Sample GUI 1"
            "window_name": "Sample GUI 1",
            "command": "echo hello",
            "components": []
        },
        {
            "window_name": "Sample GUI 2",
            // Of course, you can define a custom value for "label"
            "label": "Menu Item 2",
            "command": "echo text_box: %-%",
            "components": [
                {
                    "type": "text",
                    "label": "Some text"
                }
            ]
        }
    ]
}