PB-912 Fix language of background layer attribution

[schtibe]

(the reason I am doing this was because I fixed something small when nobody else was around, this is like the extension of that) Some notes on the fix: the backgroundLayer was copied inside the store. When one changes the language, the attributions would therefore not be updated. One solution would be to listen to language changes and trigger the update (see first commit), but IMO this would diffuse the responsibility of the data further. So I changed the store a bit so the currentLayer comes through a getter, streamlining the data flow.

Test link

[cypress[bot]]

web-mapviewer    Run #3440

Run Properties:   Passed #3440  •  0f04cff899: PB-912 Refactor map attribution for language updating

Project	web-mapviewer
Branch Review	feat-pb-912-swisstopo-source
Run status	Passed #3440
Run duration	05m 21s
Commit	0f04cff899: PB-912 Refactor map attribution for language updating
Committer	Stefan Heinemann
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Test results
Failures	0
Flaky	0
Pending	21
Skipped	0
Passing	211
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