print: Use same print operation throughout

Sophie Herold requested to merge wip/sophie-h/fix-printing into main

Use a hack to wait in print signal for the layout dialog to be ready instead of creating a new print operation with the same settings.

This works around a bug in GTK where the wrong printer gets selected (issue #243 (moved)), as well as the print dialog getting shown twice when using the print portal (part of #61).

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