Stop scrolling when switching between windows

[vilari-mickopf]

Fix #99

active_scroll.winid was present but not used, so i've used it to set current win id in before_scrolling function, and then check if window is changed in scroll_callback function.

[karb94]

Thank you for reporting #99. The active_scroll.winid is there because I am working on adding functionality to scroll windows other than the active window (one application of this is #79). This will also prevent #99 from occurring because the scroll will be tied to the winid. I will close this as I think users are not severely impacted by #99 and can wait a little bit longer.

[vilari-mickopf]

Understandable, it was just annoying and looked like an easy fix.