The export dialog was a mess and had lots of duplicated code.

This refactor splits the export dialog into 3 files:

1. export_dialog.py - Handles the actual gui dialog
2. export_workers.py - Handles the processing for generating the frames and writing them to file
3. export_formats.py - A registry of the various export formats and their capabilities.

This greatly simplifies the export dialog and should make it easier to make changes to it in the future.

The option to export a PNG sequence has also been added.