Remember at what point in time a project was fully resolved.

And use that information for out-of-date checks instead of the build
graph time stamp. The latter is updated after resolving or even building
has finished, which means we miss all changes to project files that have
happened in between.

Change-Id: I4b2f9e962c26894ce167b36ae3e562e7905d2a3c
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>