Add RTL i18n support

[izzy]

Some languages translated use RTL instead of the western standard LTR text direction. This probably needs some changes in the CSS code to actually work properly with these languages.

I assume the styles/menu.css will need at least some additional styling. There is documentation available on how to use the predefined messages with the i18n API: https://developer.chrome.com/extensions/i18n#overview-predefined