C++: Fine-tune auto insertion of '}'

Do not insert for these cases:
 * <Cursor>{
 * namespace X <Cursor>
 * if the next block is indented, like e.g.:
     if (e) <Cursor>
        g();
 * on empty line if text before looks like a finished statement or
   scope opening/end

Change-Id: Id9decc1e964a775724a929c2a3e79b5283105560
Reviewed-by: David Schulz <david.schulz@qt.io>