Added Heirarchy in Widget Classes

Added Containter class, made Label inherit Container and PushButton
inherit Label. In other words, implemeted a class heirarchy rather than
making all classes get all properties from oxide.widgets.commonprops.

Also made WidgetRegister into one class, rather than inheriting
__WidgetRegisterBasicTypes. In WidgetRegister, added
BasicBox_styleproperties and TextAndIcon_styleproperties, letting
typedicts inherit those rather than containing new dictionaries that
would need to be changed each time a new property was implemented.

Added borders and rounded corners for styling.