erp5_svg_editor: fix should autofocus editor to enable keyboard shortcuts

also prevent from going backward by pressing backspace

bt5/erp5_svg_editor/SkinTemplateItem/portal_skins/erp5_svg_editor/svg-editor.erp5.js.xml

@@ -44,6 +44,7 @@
     async: true,\n
     success: function (text_content) {\n
       var svg_canvas, frame = window.frames.svgframe;\n
+      frame.focus();\n
       if (text_content !== "") {\n
         svg_canvas = frame.svgCanvas;\n
         svg_canvas.ready(function () {\n
@@ -68,6 +69,12 @@ $(document).ready(function () {\n
     });\n
   });\n
   $("iframe#svgframe").css("border", "0px");\n
+});\n
+\n
+window.addEventListener("keydown", function (event) {\n
+  if (event.keyCode === 8) {\n
+    event.preventDefault();\n
+  }\n
 });</unicode> </value>
         </item>
         <item>