Add a "copy full CSS path" option to the inspector's menu

Issue #3

devtools/client/inspector/test/browser_inspector_menu-01-sensitivity.js

@@ -26,6 +26,7 @@ const ALL_MENU_ITEMS = [
   "node-menu-copyinner",
   "node-menu-copyouter",
   "node-menu-copyuniqueselector",
+  "node-menu-copycsspath",
   "node-menu-copyimagedatauri",
   "node-menu-delete",
   "node-menu-pseudo-hover",

devtools/client/inspector/test/browser_inspector_menu-02-copy-items.js

@@ -25,6 +25,12 @@ const COPY_ITEMS_TEST_DATA = [
     selector: "[data-id=\"copy\"]",
     text: "body > div:nth-child(1) > p:nth-child(2)",
   },
+  {
+    desc: "copy css path",
+    id: "node-menu-copycsspath",
+    selector: "[data-id=\"copy\"]",
+    text: "html body div p",
+  },
   {
     desc: "copy image data uri",
     id: "node-menu-copyimagedatauri",