parse: escape \ with a \ as well, so that we don't lose the \ when unescaping

parse: escape \ with a \ as well, so that we don't lose the \ when unescaping

If we have a file called Foo\Bar.ogg, there is no way to pass
that filename properly to filesrc in gst_parse_launch(), since
gst_parse_unescape() will just unescape \x to x.

Not cherry-picking this into 0.10 since there are apparently
apps that work around this problem and which would break if
we fixed it there too.

https://bugzilla.gnome.org/show_bug.cgi?id=673319