Add a project-specific flag that prevents saving the project

[KeyWorksRW]

Description:

It's possible to load a project file from a newer version of wxUiEditor that has properties that we don't recognize. It's likely that the various objects will display just fine, but if the project file is saved then those properties will be lost. What we need is a flag for a project that disables Save. That way a project can be loaded, but the user has to use SaveAs if they want to save the project.