Reverse the notify.conf linked list before writing

Reverse the notify.conf linked list before writing

hexchat populates the single linked list `notify_list` defined in
`src/common/notify.c` from `notify.conf` file. Each new line read from
the file is added to the list by `g_slist_prepend()` which adds it to
the front of the list. But in `notify_save()` the list elements are read
from the start to end of the list and written to the `notify.conf`. This
means everytime hexchat is opened and closed, the contents of
`notify.conf` get reversed. This commit creates a
temporary glist in `notify_save()` and applies `g_slist_reverse()` on it
and writes the contents of this reversed list to `notify.conf`. And
solves issue #2680