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)
You can already drag and drop files and folders onto Brackets, and drag and drop files in working set, and there are requests for other drag and drop features (e.g. moving files in project tree), so I think this preference name is too generic. Maybe rename it to "dragDropText" or "codeMirrorDragDrop" ?