PopUp Window / Multi-Window handling abstraction layer

[slu-it]

Currently it is very cumbersome to handle multiple windows (for example pop-ups) with either Selenium or WebTester. There should be an abstraction layer for seamlessly switching between windows and executing code. Within this issue a concept should be created / discussed.

[slu-it]

I have some experimental code in my repo:
https://github.com/slu-it/webtester2-core/tree/window-handling