Bump to gnone15-0.9.6

This bump implies new packaging rules; consequently virtual/gnome15 was
replaced by app-misc/gnome15, which contains all code in a single
package.

Plugins that requires dependencies are activated by corresponding
well-known use-flags (except the new nexuiz use-flag).

Some plugins were left disabled, since I have no idea how to make them
work appropriately (notify-lcd, ppastats and such).
Update README for gnome15-0.9.6