Refactor to add SKUI dialog

Adding SKUI involved some fairly major refactoring.

* Added constant for OPTIONS.
* Added method to get file extension (stl).
* Added method to return model name.
* Added method to select file path for export.
* Added method to export model (decoupled from UI)
* Added method to get scale factor from units.
* Removed references to 'other' entities.
* Added args to several methods to make them callable via tests.
* Removed inputbox options dailog.
* Added SKUI options dialog. ( do_options() )
* Added export selected only option.
* Updated .strings file to match dialog labels.
* Other small tweaks, fixes, and clean-ups.