facepackages/colorgrid: use FormLayout as top level

Most configuration pages for faces use FormLayout as their top element, but the one for colorgrid wraps it in a ColumnLayout. As a consequence, this is the only page where the form is vertically centered. While this does look nice, especially for a page with few options, the config pages should be consistent with each other – if vertically centering is desired, it should happen for all of them.

This change removes the ColumnLayout, making FormLayout the top level item and removing the vertical centering.

before	after

for reference:

Horizontal Bars face	Text Only face