pre-entity-component-system

This is the last revision before the extensive work was begun to introduce a
component system for game entities to Cafu.

It is expected that the development will take a long time. Although it is
intended to keep backwards-compatibility wherever possible, it is expected
that compatibility to now existing binary files (especially `cw` world files)
and to custom Lua map scripts (e.g. `TechDemo.lua`) must and will be given up
in the process.

Contrary to the already completed introduction of a component system for GUI
windows, it is expected that the upcoming work is of a long-running nature,
comes with extensive changes to almost all parts of the Cafu Engine, and is
possibly "open-ended". It is therefore developed in the `master` branch rather
than a feature branch that would run for months and years, and in the meantime
only leave `master` back at this commit, which will sooner rather than later
become old and unsupported.

Also see http://www.cafu.de/forum/viewtopic.php?p=6212#p6212 for additional
information.