sclang: fix Main.elapsedTime

we should use the system clock as this is used for scheduling

Signed-off-by: Tim Blechmann <tim@klingt.org>
include: split public and private headers & prototype libsclang interface

Signed-off-by: Tim Blechmann <tim@klingt.org>
sclang: SC_LanguageClient - move symbols out of the interface

Signed-off-by: Tim Blechmann <tim@klingt.org>