Layer export needs refactoring

[a-y-khan]

• [ ] Unit tests for exporter base class
• [ ] Unit tests for export actions
• [ ] Consolidate cut and paste exporter code for managing label values in exporter child classes
• [ ] Consolidate cut and paste code for filling in background mask layer data block in exporter child classes
• [ ] Refactor ActionExportSegmentation so action code is in run method, and not validate
• [ ] ActionExportSegmentation layers argument as vector of strings makes more sense
• [ ] Add optional ActionExportSegmentation argument values that contains list of label values
• [ ] Exporter type not set in ActionExportSegmentation::Dispatch methods
• [ ] Check possibly unused string processing code in SegmentationExportWizard

[a-y-khan]

Consider splitting up ActionExportSegmentation - the code path that takes the fully configured file exporter does not need the same arguments and the path that takes the list of layers.

[a-y-khan]

See also #70.

[allywarner]

#287