Refactor export functionality

[cbrnr]

Currently, the export machinery is a bit hacky and has grown over time. For example, there's one main function export_file in mainwindow.py, which calls specific export functions in model.py. However, these specific functions check file extensions differently with a ffilter parameter. The export_data function doesn't have this ffilter parameter and leaves the checking to functions defined in io.writers.

I think a consistent API would be nice, suggestions can be collected in this issue.