this new implementation of `waitForKeyboardHide` seems way more reliable. I executed calendar tests 15+ times and worked in 100%. (see: https://travis-ci.org/mozilla-b2g/gaia/builds/17993517)

I still did not had the time to extract it into marionette-helper, and I'm not even sure if it belongs there.. all methods on marionette-helper would suit any application, and this is very specific to gaia current structure.

@gaye your [original suggestion](https://gist.github.com/gaye/8714413) did not work but it was a very good pointer to me, I would take way longer to figure out what I had to do without it. Thanks a lot.