Revert "gedit-app: use g_option_context_parse_strv()"

This reverts commit 0e82694a539de3017aa62e09cf32f4016f3df465.

It turns out that things are actually trickier and we rely on the
fact that the original array is left alone. For now we revert so
that "gedit foo.txt" works again, but we should revisit this once
https://bugzilla.gnome.org/show_bug.cgi?id=721977 lands in glib