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.
Missing .entities on entity.entities.parent.entities
Properly embed SKUI
Add SKUI files.
Use STL_ constants
Always load the embeded version of SKUI
Use more descriptive variable names row and col.
Remove whitespace around method arguments.
Change 'w' variable to more descriptive 'window'
Out-dated comment.
SKUI needs to be embedded at the Class/Module level.
Make export SKUI window modal
Disable SKUI debugging
Merge pull request #93 from jimfoltz/feat/skui-dialog

Refactor to add SKUI dialog