File: Open single instance / Open in new instance / Revert

[jmp909]

should be fairly obvious these...

• Open single instance (settings option): only allow one project open at a time
• Open in new instance: this is actually default, but should be optional. Otherwise normal "Open" should just replace the current project
• Revert: currently I can't revert to saved version. If I've made edits and want to go back, I can't open the same file again... I have to open a new window and then open it there.