Add method for defining default style for a class

See samples/sample56.rb for a test
Merge pull request #460 from shoes/style-for-elements

Implement style method for default text block styles
prepending is handled in SlotContents now
Stop including the DSL module into Slots

* everything is executed within the app context
* introduces Clear module, since that's remaining as a common bit
  of functionality, looking to kill it somewhen though
* Significantly reduces amount of specs run since all the DSL
  specs don't have to be executed for Stack/Flow
hover/leave are defined to be defined for individual slots
Better naming and dryer mouse control listener additions
in_bounds? fix taking into account absolute positions not relative ones