Wizards: Open a relevant file in editor after the wizard run

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

src/plugins/designer/formwizard.cpp

@@ -67,5 +67,6 @@ Core::GeneratedFiles FormWizard::generateFiles(const QWizard *w,
     Core::GeneratedFile file(fileName);
     file.setContents(formTemplate);
     file.setEditorId(QLatin1String(Constants::DESIGNER_XML_EDITOR_ID));
+    file.setAttributes(Core::GeneratedFile::OpenEditorAttribute);
     return Core::GeneratedFiles() << file;
 }