Implement CM Drag & Drop

Implements https://trello.com/c/io24Byhm/910-drag-and-drop-to-move-selected-text-within-editor.
Adds a new pref `dragDrop` (which defaults to true)
This new feature is enabled by default. New features are usually off by default so as to not update current behavior, but I can be swayed by public opinion. What does everyone else think?