[Modal Window Helper] Return botton / choice

[MillhioreBT]

Pull Request Prelude

• I have followed [proper The Forgotten Server code styling][code].
• I have read and understood the [contribution guidelines][cont] before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

By returning the button or choice then we can add new fields that can be used for anything.

Example:

local button = modalWindow:addButton(...)
button.value = index
local choice = modalWindow:addChoice(...)
choice.parameter = index

I have also added (line breaks) \n in io.write messages, since io.write doesn't add newlines at the end.

Issues addressed: Nothing!