Added Moon::Context#symbols
Added RenderContainer#init_elements stub
Refactored Input::Observer
Fixed Text#align :center and :right

Since bounds returns an array, the first parameter (x) needs to be extracted
Drop Moon:: prefix for internal object creation
Make RenderContainer#init_* functions private
Migrated SkinSlice* from es-moon
Added some widget stubs

Widget
Button
Label
Textbox (the long awaited Textbox)
Window