Use FocusGained/FocusLost events to avoid showing notification when window has focus

[osa1]

The new notification feature is really useful -- however there's one improvement that would be great to see. When I'm already focused on tiny I don't want to see any notifications as the window is already in front of me and I can see everything in the tab bar. We already catch FocusGained and FocusLost events from the terminal, so this is just a matter of implementing a boolean state for whether we have focus or not, and show notifications only when we don't.

One caveat is this won't work when tiny is running inside tmux, so we should probably provide a flag for disabling this feature.

cc @meain