(Un)escape more characters

[Wormnest]

As far as I can see, polib currently only "Escapes the characters \\\\, \\t, \\n, \\r and "".

msgfmt apparently supports

Table 15-1. Escape Sequences

\n	newline
\t	tab
\v	vertical tab
\b	backspace
\r	carriage return
\f	formfeed
\	backslash
"	double quote
\ddd	octal bit pattern
\xHH	hexadecimal bit pattern

In gimp-help we have come across the vertical tab which I would like to detect when parsing with polib.

This is currently not possible because as far as I can see I cannot distinguish between a real vertical tab \v and \v inside a path (e.g. c:\\various):

• \v is not unescaped and stays \v
• c:\\various is unescaped as c:\various

[Wormnest]

It seems that gettext also is not happy finding \a. It would be nice if that could also be handled.

../../gimp-help/po/sv/using.po:617: warning: internationalized messages should not contain the '\a' escape sequence