Retrieve coordinates of current active window

[s1hofmann]

Short overview
Provide functionality to retrieve coordinates (x,y,w,h) of the currently active window

Use case
Limit actions like e.g. image search or OCR to a specific window

Detailed description
When performing image search or OCR one might want to limit the search space to a single window to reduce possible false positives.
libnut should provide functionality to retrieve the absolute on-screen coordinates (x,y,width,height) for the currently active window

[s1hofmann]

Closed by #23