CppEditor: Concatenate strings with + instead of %

operator+ is the official way for concatenating strings whereas operator%
is an optimization that needs certain defines.

The code which was using operator % is not performance critical.

Change-Id: I4c7f4310cc5708aa5f9009672d932f913a2eb59f
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>