app: Add GimpChannelBorderStyle enum

Will be used as a parameter to control the behavior of
gimp_channel_border(), and the corresponding "Select -> Border..."
action, instead of the feather flag.

Can be one of:

  - GIMP_CHANNEL_BORDER_STYLE_HARD: Current behavior, unfeathered.
  - GIMP_CHANNEL_BORDER_STYLE_SMOOTH: Smooth border, better handling
    partial selection, implemented as explained in the next commit.
  - GIMP_CHANNEL_BORDER_STYLE_FEATHERED: Current behavior, feathered.