Fix save copy callback not called when confirmation popup used, minor refactoring

[Robyt3]

When using "Save Copy" to save a copy of the current map to a new file and selecting a file that already exists, a confirmation popup is shown. However, this confirmation popup did not differentiate between regular saving and saving a copy, so the regular save callback was always called.

Add the GET parameter w=1 to the URL of this PR to show simpler diff (ignoring whitespace).

Checklist

• Tested the change ingame
• Provided screenshots if it is a visual change
• Tested in combination with possibly related configuration options
• Written a unit test (especially base/) or added coverage to integration test
• Considered possible null pointers and out of bounds array indexing
• Changed no physics that affect existing maps
• Tested the change with ASan+UBSan or valgrind's memcheck (optional)

[def-]

bors r+

[bors]

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