Add property "color-tag" of type enum GimpColorTag to GimpItem so all
layers, channels and paths can be tagged with a color.
For interoperability, use the color list from Krita which is a
superset of Photoshop's colors.
Features a "Color Tag" submenu in the layers, channels and paths
menus, a row of color radio buttons in the properties dialogs,
undo and PDB API.
As a side effect, some common code is now factores out into
items-actions.[ch] and items-commands.[ch] which adds visible, linked
and lock actions for layers and channels.
Introduce item-options-dialog.[ch] which abstracts this away and use
it from the layer, channel, vectors options dialogs. This is all
pretty ugly but better than duplicating that code three times. The
vector-options-dialog is now completely pointless but I kept it anyway
for now, let's see what unique path options we come up with.
Michael Natterer