Use window ID for target window

[MRLokop]

Is i can get HWND of window or pass it instead title?

[SnosMe]

Similar to #7

From README "You can initialize library only once", that's a very big limitation atm. It's common to restart target app but keep your utility app in tray for days, so need to add cleanup code to stop watching window, so we can change PID.

Applies here too, but for window id instead of PID.

[ErikApption]

@SnosMe I'm looking for this feature as well for making a linux screensaver inside xscreensaver. Looking to jump into your c code - do you have any suggestions for testing and developing this feature? Any test program I could use for this? (beside xscreensaver)