Use native HTML5 Drag'n'Drop for text.

Sorry, I donno how to rebase branch, so this is the new version of https://github.com/ajaxorg/ace/pull/1431 .
All issues I wanted are fixed.

This needs test and review.

Issues I would like to fix but need help:
- [ ] Wrong selection on undo of drag operation.
- [x] Autoscroll while dragging should not scroll if there is no line (scrolled past last row) or char (scrolled past last char) at cursor.
- [x] Editor should not scroll selection into view if we leave (through negligence) editor rect while autoscrolling.

Issues we can not resolve:
- Wrong ui feedback (drag cursor) in IE9+
- Safari 5 does not support dnd

Issues I do not know reason of:
- Opera blinks drag cursor (cursor icon) while dragging.

Issues I don't like or not sure:
- Sequence click interval is handled by browsers differently.

Also, in Firefox, dragenter event fires twice, so dragleave event does not get caret back. Can anybody reproduce?

Great news! All bugs I (or someone else) found in IE and reported to MS are marked as Won't Fix )))
They say those are "by design", "not reproducible" etc :)
1) https://connect.microsoft.com/IE/feedback/details/789773/ie-increments-event-detail-on-clicks-over-different-locations
2) https://connect.microsoft.com/IE/feedback/details/790344/ie-ignores-draggable-attribute-if-set-after-mousedown-event
3) https://connect.microsoft.com/IE/feedback/details/782906/setting-dropeffect-has-no-effect-in-ie9-and-ie10-standards-mode
4) https://connect.microsoft.com/IE/feedback/details/780585/dropeffect-not-working-properly-in-ie-standard-mode-ie9-ie10