app: GimpOperationTool: change values for color roles property keys

Foreground and background are GIMP concepts; the annotations on GEGL ops
should be about how the operation makes use of the meta-data; not where
it comes from in one particular application using GEGL.

old names:

"role", "foreground-color",
"role", "background-color",

new names:
"role", "color-primary",
"role", "color-secondary",