Use native HTML5 Drag'n'Drop for text.
Set `this` context of event handlers for old IE.
Fix IE native drag'n'drop issues
Improve autoscroll behavior for text dragging
Fix Opera drag image issue.
Make editor container to be drop zone
Add onMouseDrag method to the DragdropHandler
Improve native selection prevention on dnd
Fix dbl/triple click issues of native dnd
Fix webkit fake mousemove event (breaking dnd) issue
Fix IE autoscroll `drop` triggering issue

Add overlay (css ::before generated content) while dragging to prevent IE to trigger `drop` event while autoscrolling
Add autoscroll delay
Capture mouse on quadclick
Merge pull request #1599 from danyaPostfactum/nativednd

Use native HTML5 Drag'n'Drop for text.