Let user "Save As..." when document has been externally modified

Currently Okular stops the user from saving their document if it has unsaved changes and was modified externally. This makes some sense because there are now two sources of truth, and Okular cannot reconcile them itself. However as a consequence, it causes data loss since the user's unsaved changes in Okular become un-save-able. This is quite frustrating when it happens.

But this prohibiton on saving over an externally-modified document only really makes sense for a "Save" operation that overwrites the original document. If instead, the user does a "Save As...", then they can save their local changes to another file and avoid losing unsaved changes. Then if needed, they can manually compare the original externally-modified document with their newly-saved document and reconcile the changes by hand.

Accordingly, this commit avoids showing the error message box and blocking saving if the user is doing a "Save As..." to a new location, rather than overwriting the open file.

BUG: 402017 FIXED-IN: 23.04.1

(cherry picked from commit b5191a2c)