Fixes#9883
There were menus that put their entries
directly in the tab menu rather than in a submenu.
This moves them to submenus to bring back GIMP 2.10 behavior.
Stray entries added by scripts are also moved to the right
submenu.
This comes with a "colormap-delete-color" into the "colormap" action group. The
action/button will be insensitive when the selected color is used in the image,
since it is only possible to delete unused colors.
Anders Jonsson