wayland: Use right sign for workspace ID

Merged Carlos Garnacho requested to merge wip/carlosg/do-not-stick-activated-windows into master

We get a signed integer (-1 meaning "no workspace specified"), store it in an unsigned integer, check for >= 0 (of course it is!) and set as the window workspace (signed integer, -1 meaning "show on all workspaces"). What could possibly go wrong?

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