Application Window as a Source

[tsankuanglee]

This is related to #128 but instead of a pre-defined fixed portion of the screen, having the option to share a specific window would be very useful, regardless of the size and position of that window.

[GrushBrush]

This is a pretty standard feature. Why is the priority low?

[hipersayanX]

@GrushBrush the Priority tag is subjective, this feature has lower priority compared to porting Webcamoid to Android that is my highest priority right now.