Add lib/shoes/ui

Move UI-related classes into ui directory.
Move DSL-related requires into lib/shoes/dsl.
Move Nokogiri dependency to shoes gem
Add shoes-dsl as a dependency of shoes-swt
Remove extraneous gem task
Add manifests for gems
Rework gem task dependencies

It's too confusing and easy to get wrong when iterating over an
array of task types. Even though there's a small bit of repetition
in this version, it's much cleaner. And correct.