Add onDismiss callback

[gregberns]

The dialog box component allows the pop up do be dismissed without actually making a selection (isOverlayClickClose). If the user can dismiss the dialog box I want to know that they did.

Thes PR simply adds a 'onDismiss' parameter that allows the developer to hook in and call a function when its dismissed.

[deavial]

In this next release tonight:

Your onDismiss was added, but there is a better way to do this. All methods now return a Promise. So the callbacks can be omitted ie:

             this.dialogbox.pop({
            title: 'Animals',
            content: 'Which animal do you like?',
            btns: [
                {
                    text: 'Frog'
                },
                {
                    text: 'Dog'
                },
                {
                    text: 'Cat'
                }
            ]
        }).then(event => {
            if (event.button) {
                this.dialogbox.alert([
                    `You selected ${event.button.text}`, 
                    `It was button index ${event.index}`
                ]);
            } else {
                this.dialogbox.alert([
                    'Dialog was dismissed without selection', 
                    'Index for this event is always -1'
                ]);
            }
        });

The original way you are working with it will work as well. Would love to hear which one you prefer.