Iterator over focused window change on Linux

Example usage:

The following will print the window that gets focused every time the active window changes.

window can be None if there is no active window.

```rust use focus_monitor::FocusMonitor;

fn main() -> anyhow::Result<()> { let focusmonitor = FocusMonitor::trynew()?; for window in focus_monitor { let window = window?; println!("{:?}", window); }

Ok(())

} ```