Implement crash recovery/autosave

[Krakonos]

In the old redmine bug 2818, pabs suggested:

It would be nice if merkaartor would periodically automatically save to a local file any edits I made to the map. Then when it starts up it could load them again. I sometimes get infinite loops or crashes from merkaartor so this would help prevent data loss. Sometimes I need to reboot in the middle of a map editing session.

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autosave would be easy way out, as the functionality is already implemented, but the save takes way too long to do it often. I will probably implement it before I have time to implement better solution.