CppTools: Rename CppPreprocessor to CppSourceProcessor

...since it does quite a bit more than only preprocessing, as the name
suggests. We use that class to process source files in general. The
output is not a preprocessed source, but a set of CPlusPlus::Documents
with symbols.

Change-Id: I787d0f22f9f042ddf0c99e8c2f0bdb9aa7001735
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
CppTools: Clean up CppSourceProcessor

This mostly makes sourceNeeded() a bit more readable.

Change-Id: I8da40090fb499837ec56276e7a4273211920c2d2
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>