Methods defined in widgets are available in URL instances

* Shoes::App handles method definitions and subscribers
* fixes #890
Mechanism to subscribe classes/modules to DSL methods

* handled by App
* intended for internal and external usage
* .subscribe_to_dsl_methods doesn't seem like the best name
Moved DELEGATE_METHODS specs down to form a logical unit
Open class forward references no longer needed
Merge pull request #902 from shoes/widgets-in-url

Methods defined in widgets are available in URL instances

* Changelog: bugfix
* fixes #890