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/cppeditor/cppclasswizard.cpp

@@ -209,7 +209,10 @@ Core::GeneratedFiles CppClassWizard::generateFiles(const QWizard *w, QString *er
         return Core::GeneratedFiles();
     }
     headerFile.setContents(header);
+    headerFile.setAttributes(Core::GeneratedFile::OpenEditorAttribute);
+
     sourceFile.setContents(source);
+    sourceFile.setAttributes(Core::GeneratedFile::OpenEditorAttribute);
     return Core::GeneratedFiles() << headerFile << sourceFile;
 }