Work around fact that FF doesn't set modifiers on drop events

Store the modifier in the original mouse event to choose between
move and copy drags.

Issue codemirror#2556

Author: marijnh

lib/codemirror.js

@@ -3660,6 +3660,7 @@
     // Let the drag handler handle this.
     if (webkit) display.scroller.draggable = true;
     cm.state.draggingText = dragEnd;
+    dragEnd.copy = mac ? e.altKey : e.ctrlKey
     // IE's approach to draggable
     if (display.scroller.dragDrop) display.scroller.dragDrop();
     on(document, "mouseup", dragEnd);
@@ -3890,7 +3891,7 @@
       try {
         var text = e.dataTransfer.getData("Text");
         if (text) {
-          if (cm.state.draggingText && !(mac ? e.altKey : e.ctrlKey))
+          if (cm.state.draggingText && !cm.state.draggingText.copy)
             var selected = cm.listSelections();
           setSelectionNoUndo(cm.doc, simpleSelection(pos, pos));
           if (selected) for (var i = 0; i < selected.length; ++i)