Use `window.CurrentChat.emoticons` instead of API

Fixes #21 as it uses the emoticons defined on page. For integration,
simply modify the on-page emoticons and changes will automatically be
picked up.

Also fixes a regression which reverted
f5e1eb0c1e168927beb3c9653209d1d26f1da33d and
634cada0fd78edc513f46c890c1b5154e491e9b7