Improve save-as overwrite of active project

[imnasnainaec]

• In the "Save As..." dialog
  • if there are unsaved changes, overwriting the active project is the same as a normal save, so the override warning feels unnecessary in that case (or perhaps should have different text)
  • if there are no unsaved changes, then overwriting the active project is a no-op, so the override warning seems especially pointless