Backup of Mastodon files

[stefanhahmann]

User Story

A user of Mastodon recently reported that his *.mastodon file was corrupted after/during a crash of Fiji/Mastodon that perhaps was related to #295. It is not exactly clear how *.mastodon could get corrupted and currently no reproducible workflow exists that would create such a corrupt file. However, the result for the user is annoying, since if no other backup strategy existed potentially many hours of tracking may be lost.

Potential solution 1

• Save the mastodon project automatically every n minutes
  • Default of n could be 5
  • n could be set via the preference menu by the user
• The autosave would generate a projectname.backup.mastodon, after the project has been saved at least once as projectname.mastodon

Related to #310