Bug 999959 - Double clicking to add an event in day and week views. r=gaye

**WIP** depends on  Bug 1052960 work.
I don't completely understand the logic at a high-level. The `daysHolder` and `alldaysHolder` get listened to for the `click` event. Then you figure out in the hours case which hour got clicked on based off of the tap height. That much I follow... but what is the `_addEventLink` and why does it (seem to) get toggled in/out of the dom on tap?