#649 File backup related plugins notifications patch

[Pavel Nedev]

These changes have been taken from this patch:
https://sourceforge.net/p/notepad-plus/patches/635/

The following plugin notifications are added:
NPPN_FILEBEFOREBACKUP - file is about to be backup
NPPN_FILEBACKUPDONE - successful file backup
NPPN_FILEBACKUPFAILED - file backup failed