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
Extracted cleanup methods into cleanup.rb.

Will help with re-use in similar extensions.
Require and include cleanup.rb
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.
Improve binary stl file detection.

Can not rely on information is the stl header to
determine file type.
SKUI needs to be embedded at the Class/Module level.
Make export SKUI window modal
Disable SKUI debugging
Improvements to binary file detection

  * Comments to explain magic numbers 80, 4 and 50
  * rescue exceptions
  * open file in binary mode
Use literal 4 for the number of bytes read for face_count
Typos
Moved Cleanup to Utils

 * Renamed cleanup.rb to utils.rb
 * load utils.rb fri loader.rb file
 * include CommunityExtensions::STL::Utils in Importer class