Rename Export Secret keys to Backup

It happened once too many times that someone sent me his
secret key even though the filename now clearly says SECRET.

I think using the word "Backup" is better. Because "Export" is
something that you give away. "Backup" is something for yourself.

Should also help with confusion about "Export your key in Kleopatra
and send it to others."

• GnuPG-Bug-Id: T5095