Since 2.10 several new layer modes were added and other layer modes were
updated. For backwards compatibility the old layer modes are now called
legacy layer modes, which are still available.
So we:
- Moved the information about the layer modes to layer-modes-legacy, since
they were describing and showing what are now the old legacy layer modes.
- Add an image and text there to show how to select the legacy layer mode.
- Describe all the new and updated layer modes (without formulas, I'll leave
that to someone else) and add one new example image for each mode, using
a new mask image.
- Add the legacy layer mode chapter to the index.
- Removed the old multilayer example since I don't think it was adding
much and had a strange location under glossary images.

(cherry picked from commit 914ea874)

# Conflicts:
#	src/concepts/layer-modes.xml