Save an "ldb" file in the config directory.

Save an "ldb" file in the config directory.

This is updated whenever we exit a File. I think that's a reasonable
balance between updating it on every single press of Space when paging
through the file (excessive disk churn), and waiting until the whole
client exits (excessive risk of forgetting to save it at all).

Perhaps an even better idea would be to schedule a timer to checkpoint
the LDB every few minutes if it had changed.