Mobile swipes and context menu

[Emanuele Cesena]

Hi, I was testing on my iPhone and I had 2 major experience issues.

1. When holding, the context menu opens exactly on the delete node item, making it really easy to delete a node by mistake.
2. I found pretty hard adding new nodes. I'd like to propose to use swipes to support adding nodes, specifically:

   swiperight -> addSubIdea()
   swipedown -> addSiblingIdea()
   swipeup -> addSiblingIdeaBefore()
   swipeleft -> insertIntermediate()

Keep up the good work! E.