Options: use MD icons in the language editor buttons

This unifies how the buttons look in different OSs, because before we
relied in the characters being rendered by the default sans-serif font
installed in the computer.

Bug: translateselectedtext:12
Change-Id: I70e524f9664a957637f37e81465c9ec435ecc3dd
diff --git a/package.json b/package.json
index 74a3012..3e4d53f 100644
--- a/package.json
+++ b/package.json
@@ -25,6 +25,7 @@
   "license": "MIT",
   "private": true,
   "dependencies": {
+    "@mdi/js": "^6.7.96",
     "@polymer/paper-button": "^3.0.1",
     "chrome-types": "^0.1.113",
     "clean-terminal-webpack-plugin": "^3.0.0",