Saving a read-only file should open the save-as dialog

[friedl-e]

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Describe the bug
Saving a read-only file should open the "save as" dialog, very much like MS Word does in the same case.

This would be a further evolution of issue #654, to do away with the rather useless error message.

To Reproduce
Steps to reproduce the behavior:

1. Create a file, save it and set it read-only
2. Open it again
3. Click "Save" or Ctrl-S
4. Error message appears.

Expected behavior
Very much like MS Word does in the same case, automatically open the "save as" dialog.

draw.io version (In the Help->About menu of the draw.io editor):
Draw.io-Desktop 16.5.1

Desktop (please complete the following information):
Windows 10 Version 20H2

[m-mohamedin]

Will be added to the next release
Thanks for the report