[NbtkWidget] rename _get_background and add new _get_background_image
[NbtkWidget] rename _get_background and add new _get_background_image

These are two private functions to allow subclasses to access images used
in button drawing. Useful for transition effect implementations.

_get_border_image retrieves the image set as 'border-image' in css
(this is equivalent to the previous _get_background function)

_get_background_image retrieves the image set as 'background-image' in css

pwithnall/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx