Improves PopUpPanel

[Lysander]

• extends arrow-factory to take some offset-parameter alongside to the size parameter that is meant for changing width and height
• improves default styling, so that the arrow is located behind the panels content
• adds, corrects or improves API- and prose-documentation regarding the PopUpPanel usage, arrow-usage or middleware-usage.

This can be tested with the following snippet. Just paste it somewhere into the headless-demo:

popOver {
    popOverButton(classes = "mr-3 mt-2") {
        +"Click me"
    }

    popOverPanel("z-30 bg-gray-100") {
        arrow("w-10 h-10", 25)

        div("p-3 py-4") {
            +"With Arrow Beneath Text!"
        }

    }
}