Add picnic table cover quest

[FloEdelmann]

Closes #3142.

Screenshot

[westnordost]

Also add it to the "outdoor" achievement?

(Maybe the achievement stuff should be refactored so that it is defined in the quests themselves to which achievements they should count. Contributors keep forgetting to add it)

[westnordost]

Looks good! Thank you for the contribution!

Is the question wording the same as for the bus stop?

By the way, it is mostly just an OCD, but sometimes I also clean the SVG / XML with an editor, e.g. when I see that stroke properties are defined but a path has no stroke, or fill properties are defined but that path doesn't fill

[FloEdelmann]

Also add it to the "outdoor" achievement?

Will do.

achievement stuff should be refactored so that it is defined in the quests themselves to which achievements they should count

Maybe I'll try to refactor this afterwards.

sometimes I also clean the SVG / XML with an editor

I'll clean them up manually.