panel-window.c: use fallback if desired output not found

Closed Johan Malm requested to merge johanmalm/xfce4-panel:output-fallback into wayland-xfw

This could happen if a specific output name is set in the panel preferences and that output is then unplugged or the panel is run from a nested Wayland compositor. We gracefully handle this situation by just picking the first available monitor rather than refusing to start.

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