Caused by rB3d5f5c2d9a5f: Color Attributes: Add initial fill color option.

We cannot fill initial color in editmode since attributes point to bmesh
customdata, the attribute data is NULL since custom data is stored per
element instead of a single array there and there is no attribute API
for bmesh.

Questions are:

• do we disable the whole adding of color attributes in editmode because

of that?

• or just grey out the color (using uiLayoutSetEnabled), also dont

try to fill it of course?

• and explain in the tooltip this is disabled in editmode, using

UI_but_disable with a disabled_hint?

• or use poll_property to hide the color completely in editmode (also

dont try to fill it of course)?

This patch just does the second option (without the disabled hint)