Safe save option

[klayoutmatthias]

Maybe that is because more users are using KLayout on Windows shares, maybe through VPN connections as many people are working from home now ...

But I receive requests to implement a "safe save" which would not destroy the file that is overwritten in case of network problems or full disks.

Another aspect is that KLayout could keep a configurable number of backup files.

[klayoutmatthias]

The solution of branch issue-720 implements the following:

• While writing to an existing file, the file is kept in background as "original_file.~backup". This backup file is only deleted when the file has been written successfully. This applies to write errors and logical (layout style) errors.
• In "Setup/General" the number of backups to keep can be configured. Default is 0 (no backup).
• Backup files are named by adding ".1", ".2" etc. to the original file name. Backup files are shuffled - i.e. ".1" is the most recent backup, ".2" the previous etc.

[klayoutmatthias]

Fixed by PR #721.