Otherwise we get an error:

    (gtranslator:21): GLib-CRITICAL **: 23:34:51.486: g_array_remove_index: assertion 'index_ < array->len' failed

to reproduce: Open from D-L a po file with plural forms that don't match
the profile's and quickly close the 3 toasts.

Since we use g_strdup, we have to use `gchar *` instead of `const gchar`
on the msg struct.

We also move the g_frees to a clear_func otherwise they caused a double free.