add ja.po; _() some strings; no more line-numbers; update all the files

new script lang/strip_line_numbers.py strips the line numbers from .pot file comments.

Should work automagically, as i added it to lang/update_pot.sh.

Updated everything to remove the line numbers.

Now .pot file diffs are quite readable. As a result, the .pot file can be updated as frequently as necessary without the worry of huge diffs consisting mainly of line-numbers.

ja.po = Japanese

missed strings were "KILL COUNT:", "FAILED MISSIONS", "Focus gain:" (pointed out by xap).