Option to disable message dialogues

[matyalatte]

Related to #34.
It is undesirable to display a dialog after executing a command that only launches another application, such as the start command on Windows.
There should be an option to skip the dialogues.

[matyalatte]

Added an option show_success_dialog at eaa6117.
The default value is true. When false, tuw does nothing after executing a command.

Usage

{
    "gui": [
        {
            "label": "Option to skip success dialog",
            "show_success_dialog": false,
            "command": "echo Hello!",
            "components": []
        }
    ]
}

[matyalatte]

Close with v0.7.0 release.