Make the modify flag writable even if the document is read-only
Even though the code is old this code path wasn't triggered in Krita 4, so whenever you'd close a read-only document (which was modified), Krita would prompt you to save. In Krita 5, it would close the document (without warning) even if the document was technically modified -- this seems dangerous because the user can forget looking at the window title to see if the document was read-only or not.