Improve native selection prevention on dnd

lib/ace/css/editor.css

@@ -5,6 +5,7 @@
     font-size: 12px;
     line-height: normal;
     color: black;
+    -ms-user-select: none;
     -moz-user-select: none;
     -webkit-user-select: none;
     user-select: none;

lib/ace/mouse/dragdrop_handler.js

@@ -305,8 +305,6 @@ function DragdropHandler(mouseHandler) {
     };
 
     this.onMouseDrag = function(e) {
-        if (useragent.isOpera)
-            document.getSelection().removeAllRanges();
         if (useragent.isIE && this.state == "dragReady") {
             // IE does not handle [draggable] attribute set after mousedown
             var target = this.editor.container;
@@ -331,6 +329,9 @@ function DragdropHandler(mouseHandler) {
         var button = e.getButton();
         if (button === 0 && inSelection) {
             this.mousedownEvent.time = (new Date()).getTime();
+            var eventTarget = e.domEvent.target || e.domEvent.srcElement;
+            if ("unselectable" in eventTarget)
+                eventTarget.unselectable = "on";
             if (editor.getDragDelay()) {
                 // https://code.google.com/p/chromium/issues/detail?id=286700
                 if (useragent.isWebKit) {