The document will be auto saved at AppDataLocation every 10 seconds if
any change happened, and
will be deleted when document window closed normally.
Naming conversion: "yyyyMMdd-hhmmss-{guid}.d3b", file
type "d3b" means "(D)ust(3)D (B)ackup".
Each time, when app start, the auto saved files will be checked, the latest
one will be loaded automatically.
This feature is mainly for save the file before crash.
Thanks Marcus Hilsdorf for his suggestion of this feature.
huxingyi