Fix translator comments

Translator comments need to be prefixed with ‘Translators:’ (NB xgettext
is case sensitive) they must also be on the line immediately preceding
the translated string. Some translator comments were previously missing
from the pot file as they weren’t close enough to the translated
string. Also remove the leading ‘*’ from comment continuation lines as
xgettext passes them verbatim which messes up the appearance of
translator comments in the pot file.  Using a prefix remove some
unrelated comments from the pot file.

https://bugzilla.gnome.org/show_bug.cgi?id=769623