Add key/value property-key meta-data store for paramspecs

This provides storage infrastructure for additional contextual ui rendering
data. Permitting to use customized, widgets - adapted interactions as well as
linking properties to each other.

No name-space or schema; though some common practice will arise from
interactions between existing ops and GIMP.
gegl-op.h: a new way of chanting ops

The existing way of specifying additional ui contextual information for
displaying interactions for operation properties

gegl_chant_double_ui (var, "var", 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, "description")

makes the inherited hard to remember order of parameters even harder to
remember than the still hard to remember order of arguments in
g_param_spec_double. This infrastructure uses var_args and named parameters;
making the declaration concise and self-documenting - at the cost of some
programming mistakes only surfacing at runtime.

The chant prefix has also been trimmed away; replacing it with more
descriptive vocabulary.

The changes also makes it easier to subclass aribitrary gobject derived classes.
threshold: use gegl-op.h
emboss: use gegl-op.h
panorama-projection: use gegl-op.h
checkerboard: use gegl-op.h
color: use gegl-op.h
noise-rgb: use gegl-op.h
layer: use gegl-op.h
brightness-contrast: use gegl-op.h
plasma: use gegl-op.h