Configuration settings

- Add global configuration settings. Settings are defined in settings.py, where each option may have a label, description, type and validator;
- Add dialog for users to edit configuration settings;
- Make the list of fields to annotate configurable;
- Make the export file name configurable using a template. To ensure we can track which part of the metadata applies to which extracted segment image, a json file is always generatd when images are exported (named 'metdata.json') and the file name of each segment image is included in the json file.