Add a progress indicator while saving/loading data from files.

[InsertCreativityHere]

Currently we have no mechanism for calculating, or displaying the current progress while saving, exporting, loading, and importing data from files. Having such an indicator might be useful feedback to display to users when they're working with large files.

[InsertCreativityHere]

We could also use this count the number of bytes written to a file, and then we could store this value as part of the data-header, so that corrupted data could be skipped.