Remove all hacks/conventions of considering the last generated
file as project file, etc. and instead add attributes flags to
Core::GeneratedFile, giving fine-grained control of what to do
with the file. Implement static utility functions in wizards
that handle it. Add boolean XML-attributes "openeditor"/"openproject"
to the file elements used by the CustomWizard XML-specification.
Manually set the attributes in all wizards.

Task-number: QTCREATORBUG-1166