app: Modify gimp_gegl_apply_border() to take a style parameter

instead of the feather parameter.

The BORDER_STYLE_HARD and BORDER_STYLE_FEATHERED styles are implemented
using the "gimp:border" operation, as was done previously.  The
BORDER_STYLE_SMOOTH style is implemented by performing a "gimp:grow" and
a "gimp:shrink", and subtracting the shrunk image from the grown image
using "gegl:substract".

gimp_channel_border() is modified to pass either BORDER_STYLE_HARD or
BORDER_STYLE_FEATHER, depending on its feather parameter, to maintain
the current behavior.  The next commit replaces it with a style
parameter as well.

Mass parameter alignment changes to gimp-gegl-apply-operation.h.  Sigh...