Fix misuse of N_()

There are a few places where N_() is used without calling gettext()
on the returned string. However, glib documentation for N_() says:
"Only marks a string for translation. This is useful in situations where
the translated strings can't be directly used, e.g. in string array
initializers. To get the translated string, call gettext() at runtime."

Since these uses are in places where _() can safely be called,
use that instead.

https://bugzilla.gnome.org/show_bug.cgi?id=682808