Give editor a z-index

Prevents text appearing over the header.

Change-Id: Iad9712e4ef105bf37a2e166876a9d8ea77585fe7

gr-editor/gr-editor.html

diff --git a/gr-editor/gr-editor.html b/gr-editor/gr-editor.html
index 9a11cbb..9ff47d1 100644
--- a/gr-editor/gr-editor.html
+++ b/gr-editor/gr-editor.html
@@ -22,6 +22,7 @@
       .CodeMirror,
       #wrapper {
         height: 100%;
+        z-index: 0;
       }
       .CodeMirror {
         font-family: 'Roboto Mono', Menlo, 'Lucida Console', Monaco, monospace;