[Icons] Add support for gradients in processIcon
[Icons] Add comments to alpha gradient code
[Icons] Include gradient icon processing in test
[Icons] Use gradient fill for tree, menus
[Icons] Tweak generated icons to ensure symmetry

Point at the center of generated icons (when present)
was slightly off-center due to rounding issues.
This special-cases the center point to ensure it
is appropriately aligned with respect to other
elements drawn.