All babl formats now have a space equivalent to a color profile,
determining the format's primaries and TRCs. This commit makes GIMP
aware of this.

libgimp:

- enum GimpPrecision: rename GAMMA values to NON_LINEAR and keep GAMMA
  as deprecated aliases, add PERCEPTUAL values so we now have LINEAR,
  NON_LINEAR and PERCPTUAL for each encoding, matching the babl
  encoding variants RGB, R'G'B' and R~G~B~.

- gimp_color_transform_can_gegl_copy() now returns TRUE if both
  profiles can return a babl space, increasing the amount of fast babl
  color conversions significantly.

- TODO: no solution yet for getting libgimp drawable proxy buffers in
  the right format with space.

plug-ins:

- follow the GimpPrecision change.

- TODO: everything else unchanged and partly broken or sub-optimal,
  like setting a new image's color profile too late.

app:

- add enum GimpTRCType { LINEAR, NON_LINEAR, PERCEPTUAL } as
  replacement for all "linear" booleans.

- change gimp-babl fun...