To fully support storing colors as a custom property, it is necessary
to allow switching the property UI to the standard color picker button.
That means in effect supporting custom property subtype values.
Change RNA_property_subtype to look for a 'subtype' string field
in _RNA_UI and parse it as an enum value; to minimize performance
impact, only do it if the property is an array.
On the python side, allow setting some most useful seeming values
from the custom property settings editor.
Also, since some color picker code seems to run into a risk of a
buffer overruns if the array size is wrong, check the size in the
UI layout code to be safe.