Add full translation support

[tfuxu]

There is no translation template file (.POT) available currently, because I haven't got gettext to work with strings defined in code. I've generated a preliminary template file with most of the UI strings available, you can find it here: po/floodit.pot

Investigate if we can use gotext to get the same translation workflow as in Halftone.

[yakushabb]

The translator-credits string should be translatable to add translator credits.

https://github.com/tfuxu/floodit/blob/master/src/views/about/about_dialog.go#L30