include: split public and private headers & prototype libsclang interface

hi all,

i'd like to merge this patch. it does:

* split the headers into public (in include/) and private (in lang/ or server/).
* clean up the public interface for libsclang (only expose the SC_TerminalClient class)
* remove some dead code

jakob, two points for you:
* i know you have been working on a plugin interface for the language. that should be unrelated to the changes of the libsclang interface, as i've only dealt with the interface, how to use libsclang.

* i'm not sure what's the best way to incorporate QtCollider::LangClient, but maybe we can simply provide a factory function which returns a TerminalClient *, which could be a QtCollider::LangClient instance.

thoughs?

Signed-off-by: Tim Blechmann <tim@klingt.org>