Move icon related constants to InterfaceDefs.h

There will be non-BControl views to use those as well.
BControl: Move icon code to separate class BIcon
BControl: Move icon code to separate class BIcon
BRadioButton: Remove non-BControlLook code
BRadioButton: Add icon support

* Fix off-by-one issue in Draw() and GetPreferredSize(). The label was
  too close to the knob.
* GetPreferredSize(): Remove empty space to the right, when no label
  and no icon was specified. Like with BCheckBox there's still two rows
  of empty pixels above and below the knob.
* Draw(), GetPreferredSize(): Add icon support.