Debugger: Only enable C++ language if project says so

Fix a regression from 2.7: A new .qmlproject did also have C++ enabled
as a debugger language by default. Instead of reviving the old, hacky
logic from 2.7 that directly manipulated the aspect object,
the aspect now checks for the actual project language, along the logic
of the QML language check.

Change-Id: I77c9323fd59079c42874dadf4dc54b9991204d9d
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Aurindam Jana <aurindam.jana@digia.com>