Add Cilantro focus/blur events for toggling session ping

The visibility API does not consider a window to be hidden when the
window is in the background. The blur/focus handlers are more general
and trigger if the window is not in the foreground.

Signed-off-by: Byron Ruth <b@devel.io>
Merge pull request #683 from cbmi/issue-682

Add Cilantro focus/blur events for toggling session ping