New FontLayer system with a string cache.

- The system adds a usage counter for strings it renders and tries to
keep the most used ones.
- Item sizes can now be changed in UI lists.
- The renderer can be changed in UI panels.
- Several ways to change one or several avatars with one or several new
states in Screen and RendererActor.
- Fixed a bug in GLString when the size of a string is given with it (a
< instead of <=, shame on me).