Retrieve window dimensions

[s1hofmann]

This PR addresses several issues at once.

It implements functionality to retrieve dimensions of a window given its window handle (#17).
getWindows retrieves a list of all available window handles, getWindowRect returns window dimensions in form of

{
    x,
    y,
    width,
    height
}

Additionally, getActiveWindow retrieves the handle of the currently active window (#15).

getWindowTitle returns a window's title (#22)

[svettwer]

I feel like this code needs an 18+ age rating, because it's porn! 😂
The way to test the implementation E2E with a simple electron app is creative as f*** and covers all platforms by just one test file. The implementation is clean an simple. I bet more time has been invested than it seems at the first place.

But it wouldn't be a review of mine, if there were no remarks or questions. 😉