avoid full reparses of the code model every time the project changes

Currently the Cpp code model is very conservative and assumes that any change to the project needs a full reparse of all the files in the project (invalidating all previous values).

One should check it include,compiler flags,... are changed wrt. to the old version, and only in those cases invalidate the older version, otherwise do a partial update.

Related with this change the update due to project change should be done only through updateProjectInfo (which possibly calls updateSourceFiles after figuring out what to do), removing the extra call to updateSourceFiles of the project manager.