exported all strings to MastodonApi.text so they can be internationalized

[nicobo]

My use case was to change the title of the widget ("Toots") but this text was mixed with the javascript code. I've therefore taken out all text strings into the already existing MastodonApi.text object. I've updated mastodon.widget-jp.js at the same time but left the english text (as it was in the master branch)

This is not perfect, but yet a little step towards an easier i18n.