Issue #10926: NDE: Save/Load color property for filters

After the latest Color Space Invasion update, this patch adds FILTER_PROP_COLOR so that we can save and load the GeglColor property of non-destructive filters in XCFs. I followed the pattern of PROP_FLOAT_COLOR, although I used RGBA instead of RGB. Tested using the Pixelize filter.

@Just-Another-Gnumer, I believe this addition was something you were interested in? If so, feel free to test and let me know if there are any problems.

@Jehan, sorry to bother you, but is this an acceptable implementation? Ideally I don't want people to get warnings about missing properties when saving GIMP's standard filters. 😄

EDIT: Linking this to #10926 (closed) since someone explicitly asked for this property.