make QMakeProject's copy c'tor actually behave like one

make QMakeProject's copy c'tor actually behave like one

instead of initializing base_vars with the original's vars, initialize
vars itself. this has two consequences:
- there is no need to call read(0) to initialize vars
- one cannot usefully call the complex read() anymore, as that would
  re-initialize vars from base_vars

this is much closer to an actual copy than the previous "seeding with
existing project".

Change-Id: Ib007bc5b779aedb680a27329aa578f7c604a4308
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>