C++ Preprocessor additions widget.

Introducing a Widget with a SnippetEditor and a C++ Highlighter which
should provide additional information to the C++ preprocessor for a
specific file.

Change-Id: I27f9498c7e52d1493d6ea92a02a2c6d26130fe07
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>