This cleans up the handling of those settings, and makes sure that the
C++ document is setting those settings before parsing. This also
prevents lock contention in the parser, because before this patch the
document would kick of the initial parse (after opening a document),
after which the editor would come in and try to set the preprocessor
settings, resulting in the UI thread locking until the initial parse was
done.

Change-Id: Ia2e18a9667e10279f045c1c5849046ec4cfe85ae
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>