UI: Don't save defaults in oldSettings in properties

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The default settings were saved in the oldSettings variable of the
properties dialog in `86eb7ae` to be able to restore default settings in
undo/redo, but this doesn't actually do anything.
Besides this, it introduced a bug where clicking "Cancel" in the
properties dialog would save all settings, including default ones, as
well as a bug where not changing anything in the properties dialog would
still add an undo action.

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