Don't require super class

[tomlokhorst]

Allow NoticeWindow to be used without requiring super class.
This makes it hard (impossible?) to work with other libraries that also require a super class (e.g. SwiftTweaks).

Suggested solution
Put all logic in a separate class that can be encapsulated in a custom UIWindow subclass.

E.g. instead of:

class MyCustomWindow : NoticeWindow {
}

Have something like:

class MyCustomWindow : ThirdPartyWindow {
  lazy var noticeWindowManager = NoticeWindowManger(window: self)
}