Bug 612128 - Prevent the editor from modifying nodes which are not under an editing host; r=roc,bzbarsky

This patch ensures that the NODE_IS_EDITABLE flag is only set on nodes living
under an editing host.  Things like text controls which used to have that flag
previously will not have it any more.  The flag would be set on their anonymous
div node instead.  Note that if text controls actually fall under an editing
host, they will get the NODE_IS_EDITABLE flag.

This patch also makes nsHTMLEditor::IsEditable return sane results (text nodes
are always considered to be editable).

editor/libeditor/html/nsHTMLDataTransfer.cpp

@@ -311,7 +311,9 @@ nsHTMLEditor::DoInsertHTMLWithContext(const nsAString & aInputString,
     // if caller didn't provide the destination/target node,
     // fetch the paste insertion point from our selection
     res = GetStartNodeAndOffset(selection, getter_AddRefs(targetNode), &targetOffset);
-    if (!targetNode) res = NS_ERROR_FAILURE;
+    if (!targetNode || !IsEditable(targetNode)) {
+      res = NS_ERROR_FAILURE;
+    }
     NS_ENSURE_SUCCESS(res, res);
   }
   else