CppEditor: Append extra ' ' in comment continuations

...for c style multiline comments and doxygens "///" and "//!".

Task-number: QTCREATORBUG-10856
Change-Id: I84a4f95ba9e676991484e45bb0f64be996ac3be9
Reviewed-by: Ulf Hermann <ulf.hermann@digia.com>
Reviewed-by: David Schulz <david.schulz@digia.com>