Potential bug in nimproject.cpp

Noticed that while doing the similar thing in DiffEditorPlugin.

If the NimProject::collectProjectFiles() is to be called several times, it will work properly only for the first time. For the next calls m_futureInterface will stay in the "Started | Finished" state and the reportStarted() and the reportFinished() called from inside the lambda won't work properly. So reportFinished() won't cause the NimProject::updateProject() will be called.