Add AppProxy class to clean out Shoes::App

(make blocks eval in a clean context)
Merge from master
Move app methods where they belong (external/internal)
Rename Shoes::AppProxy -> Shoes::InternalApp
Rename @__private_proxy__ -> @__app__
Fix app title, resizable, fullscreen specs
Update app fullscreen specs to expect syntax
Fix app add_child specs
Fix style specs for app
Update style specs to expect syntax
Fix shape DSL method for app
Fix specs failing on #visit
Extract shared context for dsl app

This context can be used across dsl element specs for basic
initialization.
Update Radio specs to expect syntax
Update Oval specs to use shared context
Rect specs: use shared context, expect syntax
Line specs: use shared context
Background specs: use shared dsl context
Slot specs: use shared context, expect syntax
Stack specs: use shared context